September 1 is three weeks from today, though for the Chicago Cubs, some September call-ups might wait until a week after that, because the Triple-A season ends September 7 and the Iowa Cubs are still in playoff contention. You can't simply denude the Iowa roster of players that can help it win, if they're still in it.
Nevertheless, the Cubs are likely to add six or seven players to their active roster when September begins, perhaps a couple by the first of the month and then more after the Pacific Coast League season concludes.
Unlike many Septembers, there won't be many prospect callups -- since most of those are already playing for the Cubs!