You cant increase the 48 fav limit, but there is another way to handle the problem.
1: Download and install
http://www.poiedit.com
2: Find the file Mapsettings.cfg on device
3: Open this file in Poiedit
4: Delete unwanted positions
5: Save as _fav.ov2
6: Place _fav.ov2 in map folder.
Then, navigate to poi, choose _fav and choose location from there..
When doing a map update, just copy over _fav.ov2 to new map folder...