In singles, apparently, there is no limit for serving. One can serve on his own discretion. While in doubles, this privilege is not observed and the change of service takes place.
This cross service goes as; the previous receiver has to take the position of new server whereas the partner of the previous server has to take the position of the receiver.
This makes it rather certain that both teammates have the opportunity to perform as the server and the receiver. This cycle is repeated after every eight points.
For instance: A1 (first server) serves the ball to B1 (first receiver), the B1 then, returns the ball his opponent. A2 (second server) hits the ball to B2 (second receiver) who again returns this. Afterwards, A1 serves and B2 receives and returns, similarly A2 strikes and B1 responds…so on and so forth.