Maternity pants

I'm at the beginning of my second trimester and my parents are starting to feel uncomfortable. I have a few questions.

-When do I start to buy maternity pants and does your size of maternity pants change during pregnancy? Ideally I want to buy as few as possible.

-Trying to decide how long I should hold out before purchasing or if I buy ones that don't go over my belly will they fit all pregnancy?

-Do people usually buy the kind that go over your belly for the third trimester?

I'm mainly looking at yoga pants because I work from home so might as well get something as comfortable as possible.

Thanks for your help!