Add Ad Accounts in Business Manager | Facebook Ads Help Center

You can only add one ad account from your personal Facebook account into Business Manager. Instead, you can create a new ad account in Business Manager or you can request access to another ad account. You’ve reached the 5 ad account limit for adding new ad accounts to your business. There’s currently no available option to request more ad accounts.

https://www.facebook.com/business/help/910137316041095