There is nothing that has a max value for the NameMapping. However, there is something that can reduce NameMapping performance. The Extended Find feature is a GREAT feature for occasional use... but if it is over-used, it CAN reduce performance.
Something to change: Go to Tools | Options | Engines | NameMapping and turn of the "Use Extended Find whenevre possible" option. This will prevent it's automatic use but won't prevent you from manually adding it. This should help for additional mapping.
For the stuff you already have mapped, in the Mapped Objects panel (the upper panel in the NameMapping editing form), if you pan right on any mapped object, there is a checkbox for "Extended Find". If it is checked for an object, this is not ALWAYS a bad thing. However, having that flag checked means that every time TestComplete needs to look up that object, it needs to do a search down the object tree until it finds it. The more "Extended Find" you have turned on, the longer the mapping takes to update/record.
You can turn off that flag for any item but, when you do so, you'll need to map any parent objects that may have been "collapsed" by the Extended Find feature.
I THINK you'll find that, as you remove unnecessary "Extended Find" triggers, your mapping will become more responsive. This CAN get labor intensive to remove them so get your sleeves rolled up. However, it does make for a better performing tool.