How many websites can I host on one account

In practice your site count is your address count, because the entire point of the arrangement is one site per dedicated IP. A USA Class C order runs from two addresses up to eight hundred, and a European one from two up to five hundred. Nothing technically stops you putting several domains on one address, but two sites sharing an address share a footprint, which is the thing you bought diversity to remove.

Why one site per address is the default

The address is the visible attribute. Anyone can resolve a domain, see which address answers, and ask what else answers there. Put three of your sites on one address and that question returns all three at once. It does not matter how carefully you varied the themes.

So the sensible model is simple: decide how many sites the estate will hold, and order that many addresses. The account structure follows from that rather than the other way round.

When sharing an address is defensible

There are cases where it is fine, and being absolutist about it wastes money:

• A site and its own staging copy. The relationship is already obvious and nobody is pretending otherwise.
• Domains that redirect. Alternate spellings or country variants pointing at one canonical site are not independent properties.
• Sites you are happy to have associated. If two brands openly belong to the same company, separating them buys nothing.

What is not defensible is parking sites together because the addresses ran short, and hoping nobody checks. That is the arrangement you were trying to leave.

Ordering with headroom

Addresses are priced in bands, and the way those bands are calculated rewards deciding the count once rather than drifting up to it. The arithmetic is explained in Billing and Refunds; the short version is that it pays to look twelve months ahead rather than one.

The other reason is operational. Adding addresses later is a separate order and takes effect without downtime, so growth is not disruptive, but doing it in small increments repeatedly is more administration than doing it once. Ordering a little ahead of where you are usually costs less than you expect.

Managing them once you have them

However many addresses you hold, they sit under one WHM login. Accounts are created there, each one assigned to an address, and an account can be moved to a different address later without being rebuilt. That last part matters: if an address turns out to carry history you would rather not inherit, reassignment is an ordinary operation rather than a migration.

For how allocation and reassignment work in practice, see IP Diversity. For choosing the grade of separation before you order, see IP Classes Explained, and for what a shared order contains, SEO hosting in the USA.