Unfortunately, the question is impossible to answer. What you are asking is based on a lot of factors like - how heavy is the application, how many external resources it’s using, how good is it optimized, is the droplet optimized, different technologies have different limits and many many more.
I doubt anyone else will be able to answer you as well. What I always do in such cases is start with the smallest droplet and as soon as I see any high load or the application starts loading a bit slower I go for the upgrade.
Another way to get a somewhat small idea is to check how big is your VPS with your current provider and try a little bit smaller, regarding resources. That way you’ll see if you are reaching the limits on it.