Hi. Pls tell me how many clients can I connect to this antenna with good signals? Is anybody has expirience with this device?

Many.

as many as you want. 100? probably more
but bear in mind, that this specific SXT has a 24deg beamwidth antenna and is more optimized to work as a client itself. for AP mode I suggest one of the "-SA" models which have 90deg antenna for better coverage:

http://routerboard.com/RBSXTG-5HPacD-SA
http://routerboard.com/RBSXTG-5HPnD-SAr2

Of course I mean SA model with 90 degree