How do I tell a parent to call less without hurting them?

Name the limit as something you need, not something they're doing wrong. "I do better when I have evenings to decompress" lands differently than "you call too much." Frame it as a need, and you're less likely to get guilt in response.

What if my mom cries when I try to set a limit?

Crying is not the same as the limit being wrong. Acknowledge the emotion — "I know this is hard to hear" — and hold the limit anyway. If you cave every time someone cries, you've taught them that crying works.

Is it normal to feel guilty about setting limits with parents?

Yes. Guilt is the normal response to doing something that goes against the family system — it doesn't mean you're doing something wrong. The discomfort usually passes once the limit is actually in place.