Simulate WiFi Stations for testing the AP
@Passerby is right, you can't test the interesting things about having multiple stations with a single Wifi device:

max total load would be the same as if a single station was trying to max out data transfer
Can't have different qualities of channels (and even less independent MIMO channel realizations) with a single antenna (or a single set of antennas)
Can't test how collisions are handled

What you can test with a single card is how the AP and infrastructure behind handles very many concurrent connections, i.e. how well it handles user state in memory. But then we're talking 50 or 500 stations per AP, not 5; but yeah, that's pretty simple to do: for most Wifi adapters these days, the user data handling is done in host software, so &quot;all&quot; you need to do is set your single adapter into promiscous mode, tell your WPA supplicant that's the case, ask it to provision for the appropriate number of connections, and craft the appropriate association packets.
By the way, you can basically plug as many USB Wifi adapters into your laptop as you've got USB bandwidth. And any OS I worked with (Linux, BSDs) supports practically unlimited number of network adapters and thus wifi connections out of the box, so there's a simple, and relatively cheap solution: Buy a USB3 hub, buy a couple of Wifi dongles, and go wild.