There is a post here in Forum, once a member asked the same Q on a 5210 device, and the answer was maximum 8, many members claim they have no issues, my answer is you don't have issues cause all devices in your home (network) are example Apple or Android and all are new devices, problems arise when Androids, iPhones, Blackberrys and Laps want to connect at same time, network crashes, there is no info. on how many devices, call or mail support, they gonna answer 100s, let's wait for someone has done trial & error or knows the answer on in this one.
If your needs are to connect many users at same time Unifi from UBNT it's able to do 100 per AP

Try this changes on 901 v3, Read post #3 here http://forum.tp-link.com/showthread.php?74921-No-Internet-Access-with-TL-WA901ND-v3-as-Extender&region=china
Set Static ip instead of Smart IP(DHCP)
About Last line, you need to read, read, understand, really understand, the concept, to do so, go to their forum, once no doubts, then you can carry on

Heavy load lead to device freezing
Just connect less.