Is CiviCRM resource intensive?
It's tempting to discard this question as being too vague to give an accurate answer to, but capacity planning is a real and difficult challenge.

Is CiviCRM resource intensive?  That depends what you compare it with - compared to a static site, yes.

A CiviCRM site with a few users occasionally looking up a contact needs far fewer resources than one with hundreds of users using it all day long for complex reports.

Are your multiple sites likely to have similar workloads?  Will the sites be used concurrently?  What virtualisation/containerisation are you using?  What memory requirements are there besides Civi?  What other resources might be contended - like network bandwidth or storage?

You could look at the specs offered by the various CiviCRM hosting providers and see what configurations they have arrived at based on their experience of hosting a large number of mixed sites.  If you search this site you will find a few recommendations of 2GB for a VPS.

There are some critical limits, eg if the php memory limit is too low then processes abort, but mostly things "go slow" rather than fail if you try to put "too much" on a server so determining how many sites you can put on your box involves understanding acceptable performance.

In practice, you probably just need to try it and see.  Monitor your resource utilisation and perceived performance and see what effect adding another site has.

Alternatively, use a hosting provider and pay them to figure it out :-)