Salmon,
As Shiner points out; it's not just quantity that counts; it's quality.
Two hundred houses spread all over the place that produces an income of, for example, £1500.00 per month is not as good as
One hundred houses, close to each other that produces £1000 per month.
If you're just starting out, which I assume you are, leaflet and canvass and take every job you're offered that you can safely do. At first all you'll have is 'drive-betweens' and it's hard graft getting those ladders on and off your vehicle for every job. I remember envying all those well established window cleaners in my area that didn't have to do this.
At first you'll have customers all over the place, but as the months pass you'll do better in some areas and not so good in others. As you reach capacity; you'll begin 'culling' some houses/areas in favour for new and better paid work.
Each month and year the overall quality of your work will improve (as in you'll have higher paying customers in a more concentrated area) and you can of course raise the cleaning price to your current customers too.
You'll find window cleaning difficult work to begin with (understatement). Your fitness/tecnique/experience aren't there; but as your round grows, you'll find it has a snowball effect. There's more customers you have, the more people there is to recommend you to friends and neighbors. And your fitness and tecnique improves too.
But to answer your question - I'm sad enough to keep records of this sort of thing. I had:
At six months; 245 customers
At 12 months; 229 customers (Less than six months prior, but I had a good cull and took on customers too - so my income stayed nearly the same).
At 2 years; 313 customers and developing what's turning out to be quite a lucrative little round. I think I've dropped around nine customers this month. And taken on around 15.
Unfortunately I don't know how many customers I've 'dropped' to stream-line my round, but I'd guess around 100 to 200, including a fair share of low paid commercial work.