Good to see you here in in the Community. Let me share some insights about the Chart of Accounts (COA) usage limits in QuickBooks Online (QBO).
If you're not using the QBO Advanced version, any activate accounts in the COA including sub-accounts will have usage limits of at least 250.
You can check you're usage limits dashboard to show you how many items you have for each usage limit.
For additional reference, you can check this article to learn more about usage limits and what your best options are if you reach one: What are usage limits in QuickBooks Online?.
Don't hesitate to leave a comment below if you have additional questions about the usage limits. I'd be happy to answer them for you.