Maurits Who is the owner of that sub account?
That depends. At bunq you have the option for simple account share without the other person(s) getting any legal ownership (this feature is called "Connect") or you can also open up a real Joint account with multiple people having full legal ownership of an account.
Both have advantages and disadvantages. In many cases, Connect is probably enough, and has the advantage of being much more flexible. You can always add and remove people via Connect, something which isn't possible with an actual Joint account.
Here are the respective topics where you can learn more about the features:
how many sub accounts can I share with someone using easy bank plan?
The Easy Bank user can accept incoming Connects from other bunq users with no restriction that I know of (there might be some limit like 25 -- it's not really mentioned anywhere, though). They can also have one Joint account with another Easy Money/Easy Green user. So it depends if they already share a Joint account with someone else or not. In any case it's limited to one. Of course, if both users have Easy Money or Easy Green, you don't have these limitations and you can open up multiple Joint accounts.