How often does the serve change?

Under Law 2.13.3:

• Every 2 points, the receiving player or pair becomes the serving player or pair, until the end of the game.
• At 10-all, or once the expedite system is in operation, the order stays the same but each player serves for only 1 point in turn.

Who chooses first is decided by lot. The winner chooses to serve, receive or start at a particular end, and the other player gets the remaining choice. Whoever served first in one game receives first in the next.