You can install as many websites as you like, so long as the configuration of the server can handle it.
For our entry level 2GB RAM, and 1 CPU servers, we recommend up to 10 low traffic websites without advanced functionalities on it.
Advanced functionalities refer to websites with:
e-commerce.
memberships.
courses or learning management systems.
multilingual or multi-languages.
websites that has external API or integrations.
Websites with advance functionalities should have either higher or better configurations (i.e. dedicated CPU).
Same thing applies for med-high traffic websites.