How common is it for certain actors to be contractually obliged to appear in every episode of a series?

I watched a review of the TNG episode “Thine Own Self” a while ago where it was mentioned that Picard doesn’t appear until the very end after Data gets back to the Enterprise, after all the scenes of Crusher having the bridge and Riker observing Troi taking the test to becoming a commander. They suggested that Patrick Stewart may not have been around for most of the filming but they needed him to appear at least a bit in the episode because it was in the contract.

Is this a common thing? Yesterday I was watching DS9’s “Paradise Lost” and couldn’t help but notice that Bashir only has like 3 lines on the Defiant and could easily have not been there. But Quark doesn’t appear in the episode at all.

But if Armin Shimerman didn’t need to be in every episode they could easily have cut him from the episode “Duet” because the only line Quark has is him wondering whether Bajoran survivors of a labour camp like to gamble.

Maybe I’m just thinking too hard about what happens behind the scenes instead of just enjoying the show.