The business manager is not limited to just agencies. You can use it on a smaller scale as well. For example, you can have one business manager account linked to a Facebook and an Instagram account, and have different people – or different teams – using the business manager to manage those two ad accounts. You can also just run ads for one business, but have more than one payment method installed, or manage deeper analytics than you get with just Facebook ad insights.
The business manager can host a maximum of 5 ad accounts, and there is no way you can request more. You can only own one ad account, and you can be admin of up to 25, but only five can be added to the business manager.
As far as other restrictions go, you can’t add an ad account to your business manager if it has already been added as an asset to another person’s business manager. You can request access, though, which will transfer control from one account to the other. You can only add one ad account from your personal Facebook, but you can create a new ad account through the business manager that is not tied to your personal Facebook.
It should also be noted that if you create an ad account through a business manager, it will be tied to that business manager and can’t be transferred to a personal user, only another business manager.
If you don’t want to use a business manager but still want to have more than one ad account, you could do something like talk to a friend or family member and see if they’ll give you access to theirs ... assuming they aren’t using it and have no plans to use it, of course. There are billions of people on Facebook and only a fraction of them have any interest in ad accounts, so there should be plenty of people you can find who would be willing to let theirs go.