I'm not sure how big you properties are, but if they're in the 1/4 acre range, you should be able to do more than 10/day (unless you're driving 15-20 minutes between each one.) I would suggest upgrading to a bigger mower, if your lots can justify it. With a 54" Zero Turn you should be able to do more in the range of 12-15 lawns per day. This way you put your money towards another piece of machinery, which you'll have for years, as opposed putting your money towards an employee.
That's what I would do...