Other than the 16 coupons permitted on a ticket I don't know of any other limits, so long as you are within the routing rules and fare rules. If you don't exceed the MPM or violate the routing rules then it should price. Have you tried? Most OTA interfaces don't permit that many connections but odds are a travel agent can just fine if it is a valid fare and routing.

But with 21 segments on the trip I'm not sure that it can be issued on a single ticket. Possibly one outbound and one return.