[Dayrion]

Yes you can increase MAX_VEHICLES but you can't spawn more than 2'000 vehicles on your server. So MAX_VEHICLES can be redefined like you want but don't go over 2'000. Also, take this in consideration:
"if you use a large amount of vehicle models, you may notice parts of your mode become framey as vehicle models are dynamically loaded/unloaded. Keeping the number of vehicle models below 150 is a good idea."