There is no limit to sectors, and you are allowed one stopover on each ticket. If you book two one ways instead of a return, you can have a stopover each way. The constraint is that you are not allowed backtracking, your overall mileage shouldn't be too far in excess of point to point mileage, and the route has to look reasonable to the agent who is taking your call. Agents can have different opinions on the matter. I had difficulty with one route (EDI-FRA-DEL-SIN-MEL) because the agent thought that Delhi was backtracking. In fact this route adds almost no extra miles and was the only way to secure availability in F on the preferred date. It took me four agents and a supervisor to get this route. And for needless sectors, I got a redemption IST-BEY-CAI-JNB-CPT when there is a perfectly good direct flight on IST-CPT. But my route didn't add many extra miles.
On the other hand, I have had much less direct routes accepted with no difficulty - one in particular that I thought should never have been allowed. But I haven't flown it yet, so I will not broadcast it too loudly. And for needless sectors, I got a redemption IST-BEY-CAI-JNB-CPT when there is a perfectly good direct flight on IST-CPT. But my route didn't add many extra miles.
I may have misread the situation, but I get the idea that these planes fly around the world without full premium cabins - it costs the airlines little to occasionally indulge our pleasures for obscure routes on leisure redemptions and in return they get our paid business on a regular basis.