I found some information on rules, but I never found a clear answer to what should be done when a served ball touches the net.
Is it point for opponent, or should the serve be redone?
If redone - will it be redone regardless of how many times it hit the net?
According to the USATT Rules
2.9 A Let
2.9.1 The rally shall be a let
126.96.36.199 if in service the ball, in passing over or around the net assembly, touches it, provided the service is otherwise good or the ball is obstructed by the receiver or his/her partner;
Which means you must serve again if other than touching the net the service would be good. other wise rule 188.8.131.52 would apply:
2.10 A Point
2.10.1 Unless the rally is a let, a player shall score a point
184.108.40.206 if the opponent fails to make a correct service