To increase the number of bots used in singleplayer mode, go to your
\Program Files\EA Games\Battlefield 2\mods\bf2\AI
directory and open the file aidefault.ai with Notepad or Wordpad (make sure to create a backup copy of it first). Read the instructions in the file which basically involves placing the word REM in front of each of the first 5 lines starting with 'aiSettings.set', then removing the REM's in front of the next 4 lines starting with 'aiSettings.set'. Change the number at the end of the aiSettings.setMaxNBots 32 line to determine the number of bots to use (e.g. aiSettings.setMaxNBots 64 for 64 bots). Note that more bots can really stress your system and hence destabilize it, causing a crash back to desktop - particularly for those with older CPUs. For example, 64 bots on most older systems will cause a crash.
