There really is no way to answer this question.
For example, what are "basic" plugins? I'm sure my basic plugins (including some that I've written myself) aren't the same as your basic plugins. Do those plugins interact with the database? Do they function once per site visit, or every time a user clicks on a new blog post or page?
500 users per day? Are they just reading blog posts with no images? Are they looking at photo galleries? Are you using a CDN?
How many websites you can have on a VPS depends not just on number of visitors or plugins, but by how high you have set your memory limits, whether you are using caching (and this includes setting it up correctly), and so many other things that it is impossible to give a number which is in any way accurate.
But if you really need a number, then 42.