Since RTAS is host based processing, cpu bandwidth becomes the major factor. When dealing with host-based systems, the bigger the CPU the more bandwidth for performance. The number of RTAS plug-ins will vary from system to system and session to session. In essence, the more things you are asking the CPU to do will dictate how many RTAS plug-ins can be used in a session. Playback of tracks, mixer routing, plug-ins, all use CPU bandwidth. You can increase the maxium Cpu usage limit in the (Setups > Hardware) and also increase the H/W Buffer Size to give Pro Tools more Cpu bandwith and a larger buffer for maxium plugin and track counts. Please refer to the Quick Start Guide on optimizing your system (pages 7, 69, 70). You can also try using the plugins as efficiently as possible by busing tracks and putting the plugin on the bus instead of each individual track, or by recording the effects onto an audio track so that you can then remove the plugin and use the processing on another track.