I've no idea about others but my Section A will get rid of any rider weighing over 8 stone. Theoretically she should be capable of carrying more than that but she doesn't see why she should.
ps She will perfectly happily tow well over 20 stone in a trap!
Daughter used to ride an 11.2 average build Sec A, and owner had limit of 9st. Think it also depends how fit they are. I think she was probably capable of a bit more than that, she also happily pulls more in a trap.
I would have said 9st for a balanced rider. I'm about that and rode two or three when they were naughty for their little jockeys. Did jumping/hacking etc no problem and got better work out of them then a child ever could.