You're marginally more likely to see understudies at a matinee than an evening performance, because if an actor needs to take planned time off, it's probably going to be a matinee (although more likely it will be a Sunday matinee than a Saturday matinee).

How often does an understudies perform?

Understudies are on-stage, every night, in a specific [ensemble] role, and they cover principal roles. Swings are off-stage and cover ensemble.”

Do understudies ever perform?

Coined in 1874, the term understudy has more recently generally been applied only to performers who can back up a role, but still regularly perform in another role.