My recommendation is to try out cooldowns if you haven't already and see how well that works.
I'm hesitant to implement limits because I'm not sure it's the best mechanism for what people are usually after. It requires significantly more code to track the lifecycle of the tracked vehicles, including across server restarts. It also does not prevent abuse depending on which plugins the server is running. For example, Portable Vehicles allows the player to pick up vehicles they own. This would kill the vehicle entity and therefore deallocate the vehicle from the player's limit, allowing them to own and pickup as many as they want. Cooldowns solve that case better.
I understand not every server has plugins like Portable Vehicles, so it wouldn't be fair to constrain the features of the plugin based on a balance issue with a single plugin. That is, there's no reason limits can't be an option. However, it's also my opinion that cooldowns are more realistic and vanilla-friendly. For example, there's nothing usually stopping a player from amassing as many vehicles at their base as they want, but it takes time to acquire them. If they can acquire them, why shouldn't they also be able to own them all?
I'm not completely opposed to adding limits. I just want to make sure it's solving a valid problem that cooldowns don't already solve just as well or better. Can you tell me more about which plugins you are running on your server and why you think limits are a better solution for your use case than cooldowns?