You've got two options at your disposal. One is "official", and the other requires some very minor (but easy) hacking. Officially, the game defaults to 250 units, but is customizable up to 500 within the limitations of the game's original programming. To change the unit cap up to 500, do the following (skip if you want higher numbers):
1. Go to /Applications/Total Annihilation Commander Pack.app/Contents/Resources/drive_c/Program Files/GOG.com/Total Annihilation/. This is inside of the application's "package contents", so if you're going in manually, you'll have to right-click on Total Annihilation and choose "Show Package Contents" to get to the directories within.
2. Look for "TOTALA.INI" and open it in TextEdit. If the file doesn't exist, just create a new TextEdit document, go to the "Format" menu, change it to "Plain Text", and then save it into the Total Annihilation game directory (the path in step 1) with the name "TOTALA.INI". Edit the contents of the file to match this:
[Preferences]
UnitLimit = 500
NoDirectSound = 0
UseWindowsSound = 1
There are spaces on both sides of the equals signs.
3. Start the game and confirm that in a Skirmish round, the unit cap is now 500. Single player will still have limitations particular to the map itself.
If you want to change the unit cap to a higher value (I prefer 1500), I just posted the instructions today. Links seem to be disabled, but just look for "How to Directly Edit the Unit Cap (Mac & Windows)" in the Total Annihilation series forum.
Short version is that you can use a free hex editor to actually edit the Total Annihilation executable (TOTALA.EXE), changing the hex addresses that govern the unit cap. The highest you can safely go is about 5000, but it becomes EXTREMELY resource-intensive, and Total Annihilation does this thing with its physics engine where more units causes the pathfinding to become simplified; units might not find their ways around obstacles as easily.
