What happens if I order the wrong number of IPs

Nothing that cannot be corrected. Address counts are not fixed at signup, and getting the number wrong in either direction is a billing adjustment rather than a rebuild.

If you ordered too few

This is the easier direction. Addresses are added to the allocation and become available to assign, without the running sites noticing. Nothing has to be moved and no site goes down while it happens.

The only thing to watch is which range the new addresses come from. If the point of the exercise is separation, additional addresses inside a range you already use do not achieve much. Say what the addresses are for when you ask, so the allocation matches the intent rather than just the count.

If you ordered too many

Unused addresses can be released. They stop being billed from the next cycle, and any address still assigned to an account has to be unassigned first.

There is one thing worth thinking about before releasing: a released address goes back to the pool and may later be issued to someone else. If a site was ever served on it, and you might want that address back, releasing is not reversible in practice.

If you ordered the wrong kind

Ordering addresses spread across ranges when a single range would have done, or the reverse, is the more consequential mistake and it is still fixable. The correction is a reassignment rather than a new order in most cases.

What matters is being clear about the risk you were managing. Separation is a defensive measure, not a ranking input, and the amount of it worth paying for depends on what you are protecting against. That question is worked through in matching IP class to the risk you are managing.

Moving sites between addresses afterwards

Reassigning a site to a different address is a change on the account and a DNS update. The site itself is untouched. The delay is DNS propagation rather than any work on the server, so lowering TTL beforehand shortens the window.

Keeping a record of which site sits on which address saves this from becoming guesswork later, and a written record of what sits where is what stops it becoming archaeology.

Getting it closer to right first time

Work out the mapping before you order rather than after. Sketching which sites go where usually changes the number people were about to buy, and it takes less time than fixing the allocation afterwards. That exercise is in mapping sites to addresses before you order.

If you are unsure, order for what you can map today. Adding later is easy; explaining a large allocation you never used is harder.