Key question. In some cases, the more the clients, the worse the service. Too much work for voice imaging talent can make for slow script turnover, slack reads and an overall bored, uninterested announcer.
Just because a guy/gal has a ton of clients doesn't always mean they are exceptionally good or will provide you with great service. Also, consider the fact that they may be voicing in a market close to yours, or a network television station broadcasting into your area. Keep in mind that while they may not be in your area now.....they could be tomorrow, especially if their rate is cheap.
Always inquire about how many clients your voice has and where they are located.