Hi!
I need to create wireless network for warehouse 40 (w) x 100 (l) x 8-10 (h) (in meters). Can I use Netgear (for example) PoE (802.3af) switch to connect to AP's? What technology I should use (mesh, one SSID + different freq. or somethin else)? How many AP's I need?

this information is not enough
you have to tell about the concurrent users
and also here you can get help related to mikrotik products only so don't go offtopic

About 20 users with Android devices. I think that 2-5 Mbit/s is enough for one user.

if there is very low interference in that area and assuming it to be open space not rooms
about 1 access point for each 1000sqmt you will need about 4-5 access points