Yes, you can use your downloaded themes in as many sites as you need, belonging to the same customer. There are no limitations on the number of domains/seats where the themes are installed.
Per our license:
You may install the theme on unlimited production websites, belonging to either you or your client, as long as using the originally downloaded theme (with no modifications), or one single modified version of the theme. Multiple modified versions are not permitted. A new purchase has to be made, to use the theme in more than one modified version.