Shared Hosting

I'm sure the host will let you know when it's time to upgrade.

Your question is too general. Every server and account has different specs.

Standard html and php is no problem at all.

If you use databases is when you need to get more than shared.

depends what you are running. a bunch of blogs, 20+, a tube site, not so much..

I have clients running in the triple digits on a shared host ... it depends on what the site is, does it use a database, how much traffic ...etc etc ...

YMMV also by the expertise of the host, and if they are able to help you optimize your sites ... and how oversold the server is to begin with.

If you are on a shared server that is severely oversold, then you probably will not get as many sites on it as you would a server with fewer clients.

There are really to many variables to give an honest answer to that, but hypothetically, if you think your plan should be able to host more sites than you are, and your host is trying to force an upgrade on you that you are questioning if you need, then its probably in your best interest to shop around a bit.

my advice to all people buying shared hosting to host lots of sites is to buy Reseller hosting or VPS (vps only if they know what the fuck are they doing...)
that will give them better overview of their sites and their usages... also there are less overselling than with shared accounts...

cPanel/WHM is also a must...
I can recommend Certified Hosting reseller accounts... They simply ROCK!!!