Hello! Finally got to look at this.
Are you seeing this repeated error in your console?
...Because that's the cause of this issue! Quads, font changes, the texture tag, the material tag... anything that causes STM to generate a new material will also generate a new submesh for that material... So for Unity UI, there seems to be a hard limit of 8 submeshes allowed on a single mesh.
Quads sharing the same texture will *not* create an additional material/sunmesh for each quad since they can share, so that is the ideal solution. It means less materials are being used on your project, too.
If this is completely unavoidable, look for "STMPagination" in the same scenes. (I've really got to upgrade this to the Utility folder already, it's a pretty steady feature), where you could lay out multiple STM objects and have them automatically feed into one another, While only needing to maintain the first one in the chain.
Edit: I don't know why my text got formatted like this after I inserted the image. Oops