Welcome to AFF Rach1616. Although there is no stipulated maximum number of surface sectors, the practical reality is that you can't have more of them than the 5 limit number of stopovers i.e., each surface sector has to be attached to the stopover city preceding it. There is no 2 surface sector limit for either phone or online bookings. The multicity tool has a meltdown if you try and enter more than 5 flights so I usually don't search for any more than 3 flights at a time, andonly rerely will I try and search for 4. That applies whether there are surface sectors involved or not and may explain why you are getting error messages.
Suggest you post your intended itinerary with details of your intended OW flights, transits and stopovers, and at which points you intend to have surface sectors. Doing that will make it much easier for people to scrutinise your trip and make suggestions.