I was also afraid of changing this number, i used almost all the trainers not featured in the game, 88 free... from 742
So far all my previous and new alterations are ok after the expansion.
make a backup just in case : P
1 use windows calculator in programmer mode, select decimal i used 900 trainers, select HEX get the HEX number (my case) 384
2 Advanced Trainer > Trainer> Advanced >Trainer Amount , put the HEX of 900 that is 384
Now Here is the problem (don't know why but "Needed Bytes" won't update until you put any offset on it ), just repeat the old offset to get the needed bytes and search for a new offset
3 get the update Needed Bytes for me was 8CA0 or 36040 bytes
4 Open Free Space FInder > Needed Bytes: 36040 . Mark Search from the beginning of the rom" and search , get the offset
and put it on Advanced Trainer New Offset, if its right the OK button will be enabled.