Festus Arthur, who has joined from Hull on a one-year contract with an option, became the latest addition announced by The Shaymen on Tuesday night, joining fellow new faces Jordan Keane, Millenic Alli, Angelo Capello, Sam Smart, Tylor Golden and Jamie Stott, Emmanuel Dieseruvwe and Jack Hunter at the club.
Millington is expecting to make three further new signings before the start of the season next Saturday, which he says will complete the squad.
"Then what we'll look to do is leave a bit of space so that, at some point in the future, if we need a bit of cover or we need to strengthen in any areas, we'll hopefully have the ability to do that," he said.
When asked if he would have the budget available for further additions, Millington said: "Depending on what it requires to bring these three in, then hopefully."
Millington says he would have liked to have been a bit further down the road in terms of squad numbers by this stage, but that he isn't far off where he'd want to be.
"In terms of quality, I'd argue the quality that's come in is probably better than we could have hoped for," he said.
"We're ambitious when we try and go out and attract players, but we're also realistic as to where we sit in the pecking order.
"But I think we've overachieved in terms of the quality we've brought in, we've brought in some really fantastic footballers."
Millington added: "The structure we've had over a number of years has been to look at young up and coming players who need a platform, we've looked at steady National League and National League North performers, and then we've looked at players who we believe have the ability to be above the level and are of a standard that maybe the others should be aspiring to be at.
"So with those three brackets, I'd say the quality within them is better now than it has been in recent years.
"We've had some fantastic footballers come through the club in the last three years, but I'd say the potential of some of the younger players we've got now is excellent.
"We've got a really solid middle bracket of performers who are of the level or higher, and along with some of the new signings we're hoping to get done, we've got players who arguably could be in the Football League already."
Millington believes the level of quality coming into the club has equalled that heading out.
"I think the quality we've brought in is of a standard that would compete comfortably with the team we had last year," said the Town boss.
"I think the difference last year was we retained an awful lot of players from the previous season, so that chemistry was already there.
"What the fans will need to be aware of is that it takes time to build that chemistry, so it might not be on show immediately, but what we have got is a quality of player who will compete in this league at the top end."
The latest first-team player to leave The Shay was midfielder Kieran Green, who joined former Halifax team-mate Niall Maher at League Two side Grimsby.
Millington says the club did receive a "very good" fee for Green, and that the deal did include add-ons.
"I think the first thing to say is to recognise Greeny's contribution to the team over the last two seasons, he's really grown as a player and a person in his time at Halifax," Millington said.
"We're a club who want to compete, we want to get promoted, we want to be in the Football League, but also we want to demonstrate a pathway for players who come here, that they can move on if, and its a big if, the clubs interested in them can demonstrate a real desire to take them away.
"Grimsby, over a period of time, came to a point where they showed that desire and we felt that with the players we had coming in, it afforded us the ability to bring in a player of Jordan Keane's quality and get him nailed down to a longer-term contract than Kieran, so it represented an opportunity to secure the future of the club by getting a player of Jordan Keane's calibre in for a longer period, which we may not have been able to do had Greeny still been here.
"It was clear we would have lost Greeny at the end of this season anyway, so it seemed like the sensible thing to do in terms of sustainability, not only for the squad, but also for the club."
Another midfielder to depart the club was youngster Sam McLintock who left without having played a first-team game at The Shay.
"We just didn't see a pathway for him," Millington said.
"Sam, the age he's at, needs to be playing regular first-team football and we didn't see him ahead of the likes of Harvey Gilmour, Jamie Cooke, Kian Spence, Matty Warburton, all players who play in his position.
"And we just felt that at his stage of his development, it was fair for the lad to be allowed to go and play first-team football."
Recent signings Millenic Alli and Angelo Capello both started in Tuesday's 2-1 friendly defeat at Fleetwood.
Millington confirmed both had signed on one-year contracts with an option.
"I've been aware of him for a number of seasons," the Town boss said of Alli, "we were watching him when he was at Ashton United but Stockport beat us to the punch and took him away.
"He's very fast, very exciting, tricky, he'll create opportunities and stretch opposition.
"With the style of play we're trying to create this season, he fits in perfectly.
"He can play off either wing and can also play down the middle but we've brought him in mainly with a view to him playing as a left-winger."
And on Capello, Millington said: "We were alerted to Angelo by a Championship club last season when it became apparent he was probably going to be released by Sheffield United.
"It was a coach at another club who made him aware, and we've been tracking him for some time.
"We had him in on trial and he's just so exciting. He can play in pockets, he can play in the hole, in the number ten role, he can play in midfield, he can play off either wing.
"He's got ability that will help us unpick defences when they sit deep and when they get into a low block against us.
"He's certainly got an awful lot of potential and I think, over his time here, we'll see him develop and become a really important player for us."
Millington added: "They're ready and they're capable of playing in the first-team but there's an awful lot of competition for those places as well.
"But they have been recruited with a particular style of play and a particular job in mind, so they will expect to play some games this season."
Millington confirmed that, of the new additions still to come, one is definitely a striker.
But the Town boss ruled out a move for Bradford City defender Reece Staunton, saying: "We had Reece in to have a look at him and see whether or not he fitted into our style of play.
"He's had other offers from Scotland so we won't be moving forward with him."
Millington also said former loanee Elliot Newby won't be returning to The Shay.
"My understanding is he's in the plans of Dave Challinor at Stockport and they have a role for him, so he's not available," Millington said.
Uncertainty surrounds the future of winger Jamie Allen due to his participation in the reality TV series Love Island.
"The situation is as has been announced by the club, that Jamie made the decision that he wanted to go on Love Island and we, as a club, will review his position once he returns," Millington said.
"I think it would be unfair to discuss it further in Jamie's absence but in the fullness of time, I expect all the information will come out."