Hello Tuan,
For this deployment size you might also want to look at the RFS4000:
https://cloud.kapostcontent.net/pub/6df18969-455c-403d-942e-885e210d861b/rfs-4000-data-sheet.pdf?kui...
If you are looking for a hardware controller then your options would be either the NX5500 or RFS4000.
In regards to your roaming question, there are 2 scenarios to consider as follows:
1 - All APs are in the same broadcast domain (layer 2): Roaming will be seamless between APs (no dropped connections) in the absence of a controller.
2 - APs are in different broadcast domains on a routed network (layer 3): There will be a slight disconnect when roaming between APs in different broadcast domains in the absence of a controller. In this case a controller is needed should seamless roaming be a requirement.
Regards,
Chris
Christoph S.