I do 20 a week right now. It gets dark by 7 here and we (me and my helper) typically start at 3:30. I can get 9 done in that time (.5-1 acre lots) before it gets dark with tight routes. I have my helper ride the Z, I take on the w/b w/ sulky and then the first guy done trims, the next guy blows.
I plan to add another 15 next year.
One word of advice is try out different things to become more efficient. Between 3:30 and 7:00, we would typically get 7 done by staying on the same lots. I have multiple stops where I have 2 or 3 accounts right next to each other. One day I dropped my guy with the Z off at a stop with 2 or 3 accounts and I went around the block and knocked out a different account. I came back when I was finished and started to trim while he was just finishing up mowing and then he blowed. That night we finished 9 in the same amount of time. Logically thinking, it should be slower but it turned out to be much faster. Give it a try.