How calculate and chose the right BJT that comands a relay
if I change the relay, Ic increases or it's fixed?


Different relays will draw different coil currents at their designed operating voltage.
Check the datasheet for your relay.

50 mA is a reasonable number for a low power 5 V relay, but pretty low if your relay is rated for more than ~2 A switched current. 

To maintain your 5 mA base current limit you might need to either use a Darlington configuration, or change from a BJT to a logic-level MOSFET.