Are bryophyte sporangia multicellular?
In Embryophyta (land plants), including bryophytes, the sporangium is usually a multicellular structure.

Perhaps you meant to ask about the number of spore mother cells (SMCs) in each sporangium? That varies across groups. In bryophytes, each sporangium has many SMCs, and accordingly produces a large number of spores. (Contrast this with angiosperms, where a megasporangium [called an ovule] has only one megaspore mother cell.)

References and further reading:

https://courses.lumenlearning.com/boundless-biology/chapter/bryophytes/

https://www.britannica.com/science/plant-development


Image attribution:
By LadyofHats. (Public domain;
https://commons.wikimedia.org/wiki/File:Hornwort_structures.jpg)