It's great to have you here, @dennisfast,
Yes. You're right. All active sub-accounts are counted in the 250 limit for the allowed Chart of accounts usage in QuickBooks Essentials.
For further insights about usage limits, I recommend the following articles: What are usage limits in QuickBooks Online?
If your business needs more than the allocated number of registers, you may upgrade your plan to QuickBooks Advanced, which allows unlimited account entry. Here's how:
If you have other questions or need further assistance with this topic, let me know in the comment section below. I'll be glad to help.