Bre Tiesi says she doesn't have many rules in her relationship with Nick Cannon ... but she's drawn one line in the sand -- no more babies!

The reality star and social media personality made the assertion on Angela Yee's "Lip Service" podcast ... revealing she's not trying to control Nick, but she can't be with him if he has another child.

If you don't know, the pair are in an open relationship ... so Bre doesn't mean she has an issue with Nick hooking up with other women. She just thinks he's already spreading himself super thin as a dad.

Bre says that -- as a mother of one -- she already feels like she doesn't have nearly enough time for her kiddo ... so she can't imagine being a father of 12 like Nick is.

BTW ... Bre also makes it clear no one should assume she's gonna be the one carrying Nick's 13th child if he does happen to have another one -- because she's doing everything she can not to have another one.

Nick Cannon might be a super dad ... but Bre sounds super serious. No more kids!