Should each site get its own cPanel account

As a default, yes. The reasoning has little to do with speed and everything to do with where the walls sit.

What an account actually fences off

Sites lodged in the same account hold in common:

• One allowance for processor, memory and simultaneous work.
• One PHP version, decided for all of them together.
• One filesystem, so each can read the others.
• One allowance for space and file count.

Sharing those is perfectly reasonable when you chose it. The trouble is that it is normally inherited rather than chosen, arrived at because adding a site to an existing account was quicker that afternoon.

A rule of thumb

1. Give your heaviest sites accounts of their own, so their bad days stay theirs.
2. Split anything with an unusual platform requirement, so one antique does not govern nine modern sites.
3. Split anything that must be unable to reach its siblings. Separate folders inside one account is not a wall.
4. Let the small and quiet ones cohabit. After caching, twenty brochure sites together are genuinely undemanding.

A quick test

Picture the noisiest member of a group having its worst day of the year. If the others suffer, the grouping is wrong. If nothing else notices, it is right.

What this question does not settle

How sites are grouped is about resources and isolation. It says nothing about how they appear to the outside world. Fifty sites still need fifty separate addresses to present fifty separate footprints, no matter how the accounts are arranged, and each site here receives its own non-consecutive address with private nameservers regardless.

Confusing the two leads either to buying accounts nobody needed or to assuming a separation that was never there.

Can it be changed afterwards?

Does more accounts cost more?

What you are buying is capacity and addresses, not account records. Splitting an existing workload across accounts changes how the allowances are divided rather than automatically increasing the bill. If you want more total capacity, that is a separate decision.

Is there a number I should aim for?

No, because the useful measure is workload rather than site count. One demanding shop can ask more of a machine than forty static brochure sites. Group by behaviour, not by arithmetic.

What if I am inheriting an existing setup?

Start by listing what is where. Very often the arrangement was never decided at all, and simply seeing it written down tells you which two or three sites want moving. We can go through it with you if it is a large estate.

Yes, though it costs an afternoon and a short pause in publishing. Deciding before the first order is simply cheaper. See Network Planning.