As mentioned by others, either you can host one website on the server (or even zero, depending on how big the individual website is and the resources it consumes) or you can host 10,000 HTML static websites on the server... There are many factors that you need to consider:
1. The provider being used, are they overselling their nodes?
2. The software configuration on the VPS (are you using a control panel? is the software optimized and configured right?)
3. What software is powering your websites? Wordpress? If so, is it optimized and configured correctly?
4. How much traffic does each website see?
5. Is the website mostly static content, or is it database intensive?
6. Did you optimize the MySQL configuration to the nodes specifications?
There really is no way to determine how many websites will fit on a specific server until you actually start to notice performance degradation. The benefit of a VPS is that you can upgrade/downgrade/resize the node at any given time, benefiting you in such a way that once you do start to see such performance degradation you can simply upgrade your package. Your best bet is to find a hosting provider that works with you to ensure that you have ease of access, also maybe check out a Managed VPS? If you don't have much experience in running a server and keeping things up to date then you may be in for some trouble, it's always good to have a helping hand kicking around to assist you when necessary. Sure, the cost is a little bit higher (depending on the provider) but it may be very beneficial on your part as this gives you room to learn as well.