Each stem is usually a single plant. Occasionally one rhizome may throw up 2 stems. So what you have is a healthy clump or group. After they are all done flowering and setting seed, you could carefully dig them up and space out the rhizomes a bit.

Very nice!

Wow, there must be a lot of rhizomes down there. Thanks!

Love those trilliums. You are lucky to have such a nice clump. Thanks for sharing the photo.