Does PROFIBUS DP communication use connection resources?
PROFIBUS DP communication in a master-slave system does not use connection resources as long as data is exchanged exclusively via load and transfer commands. The number of your DP slaves therefore does not detract from your connection resources pool. Nor do you have to take the DP communication into account in the "Communication" tab (available in many SIMATIC S7-300 CPUs) of the CPU Properties dialog in HW Config when reserving resources.
In all other cases such as S7 Basic communication, S7 communication, S5-compatible communication etc., where data exchange is via communication blocks, you must have connection resources available.