You can run 10 active campaigns simultaneously, be it for the main account and/or for subaccounts.
Example 1: You have one main account, so you can run 10 campaigns for the main account.
Example 2: You have two subaccounts, so you can divide the 10 campaigns over the two subaccounts which they can manage themselves.
Example 3: You have one main account and one or more subaccounts. The main account manages the 10 campaigns which the subaccounts can use.
Example 4: You have one main account and one or more subaccounts. The main account manages 5 campaigns and one subaccount manages 5 campaigns.