The Scot has made five new signings this summer ahead of the club's first Premier League season.
The latest deal for Sevilla midfielder Gary Medel smashed the club's transfer record, but Mackay admitted that Cardiff's spending has not finished yet.
"We're still looking and trying but it will only be players I feel that are going to improve the group," he told BBC South East Wales.
"Like everyone else, with three weeks still to go in the window, [we are] looking to see if there's anyone that can improve our squad.
"We've got various irons in the fire at the moment and they're all moving along and progressing at different levels. If we could at least get another two, then that would help."
Cardiff face West Ham United for their season opener next Saturday.