That's where the human architect comes in (for now at least). We'll try to think of features that would have the least amount of conflicts when merged back to main. We usually max it at 3, and have a senior dev handle any merge conflicts.
That depends on how decoupled your codebase is and how much overlap in the areas being worked on by your agents are. If you have a well architected modular monolith and you don't dispatch overlapping issues, it's fine.