Is there some kind of vehicle limit ? Or can I surround an enemy base with spawn points ?
You can only have one vehicle spawned at a time, however, multiple people can spawn Sunderers and spread them out enough to effectively surround a base with spawn points.
When a Sunderer is deployed, another Sunderer cannot deploy nearby. This is signified by the bubble around the Sund when driving your own.
You can spawn a sunderer, park it next to the vehicle spawn pad, spawn a second sunderer, get back into the first one. You will now own the second sunderer, but he able to drive the first one, park it, deploy it etc. Anyone who gets in the first sunderer will now own it. So while it is technically possible to have two in operation, it's not actually that useful