It's really up to you. Payments are probably the biggest determining factor.

Payments are handled by account. So, if you have 10 clients in one account, they'll all be pulling from the same funds and the same funding sources. If you're footing the bill, one account is probably fine. 

If you want them separated, or need to allow clients to login to add payment details, you'll want a separate account for each.

You may also want to consider logins and how your agency manages clients. Each account has just one login. So, if you have specific managers for each client, and you like to keep their activities and access restricted to their assigned clients - you'll want to create separate accounts for each client.