Varies dramatically by situation, of course, but I'll lay it out as best as I can. TL;DR: It seems like about 300 customers is kind of the sweet spot if you're going for a lifestyle business - few enough customers that you can probably manage it yourself or with a little part time help, and enough to pay yourself around 6 figures.

Breakdown: Fiber circuit, up to 1gpbs, 2-3k/month. Should support up to 500-700 customers at least. Backhaul - $2k-12k each. Usually need about 1 for every 300-500 customers depending on network topology. Access Points - $200-$600 each. One for every 30-50 customers. Switching / routing hardware - $300-$1200 should cover up to 500 ish customers.

Per customer expenses: $200-$350 - customer CPE equipment and install accessories (cat5 cable, etc.) $3-$10/month - billing/customer management software, 3rd party phone support (if you do that), other misc stuff.

Then you need a ladder and tools and a vehicle that can carry them for doing installs and maintenance.

Obviously the risk depends on the competitive landscape. I've seen some people start by selling an apartment complex or office building or similar on an exclusive / semi-exclusive deal to provide service to the whole complex (which you can do considerably cheaper per customer than the above numbers) and use that to cover the basic expenses and expand from there, that's a good way to do it. In most places in the US people are unhappy enough with Comcast et al to give something else a shot. One issue is that people are usually in year contracts so you have to wait that out. If you have an existing consultancy or some community goodwill that you can leverage in to a customer base that decreases the risk, obviously.