From Hooman's notes:
Player Unit limits:
-------------------
Play Veh Bldg Tot/P Tot
- --- --- --- ---
1 200 250 450 450
2 160 175 335 670
3 115 100 215 645
4 90 75 165 660
5 77 50 127 635
6 65 48 113 678
Per player unit limit isn't hard set, and can be exceeded
by producing structure kits before deploying, or starting
production on vehicles before the limit is exceeded.
Current unit limit: 1023
[Includes mining beacons, fumaroles, disasters, etc.]
maxNumGaiaUnits = 1023 - 678 = 345
[assuming players don't build past their individual limits]
These values are pre-1.3.5; starting with 1.3.5 they were doubled. The table is soft unit limits.