The Bluebirds are aware of the need to reinforce after stepping up into the Premier League, and have spent big in an effort to cement a standing among the elite.
Three of their additions have broken the club's transfer record, with Steven Caulker, Gary Medel and Andreas Cornelius demanding sizeable fees.
There is, however, still money available and a few more days in which to get new recruits on board.
Mackay admits he will be looking to dip back into the market before Monday night, but doubts he will do more than one more deal.
He said in the Daily Telegraph: "We're trying hard and we're going to make sure we do everything we can right up to Monday.
"We would like to at least get one more in, but it's not easy because they have to be players that bring more quality, rather than the quantity of bringing three or four people in the last week and just filling up a squad."