Sometimes 2 when my brother and his girlfriend had their little girl my brother and I were both in the operating room as she had the c-section
Depending on the facility... either one, or none. Maintaining the sterile environment is first priority, second priority is making sure the doctors and nurses have all the room they need to do their jobs safely.
Only one, that has been my experience each time. The first time I let my mom come and the second time my husband came in.