。
The lawyer has many clients.
Literal
Lawyer [topic-は] many [gen-の] clients [obj-を] is-holding.
持っている is the 'have' equivalent for things one possesses or counts as one's own — unlike ~にはある/いる ('there exist for X'), which describes existence. 持っている works specifically for alienable possession (objects, clients, ideas, opinions). Note the resulting-state reading of ~ている: 'is in possession of.'