Although there can be a lot of guesswork involved when making any projections, if you do enough research - you can determine whether your business plan is likely to be viable. When somebody is interested in starting a carpet cleaning business, most people on these forums tend to recommend a carpet cleaning training course as a starting point. This is all well and good and I certainly would not discourage this - but on its own becoming a good carpet cleaner will not guarantee you success. I would focus my efforts on becoming a really good businessman first and a carpet cleaner second. If you have never run a business and don't know much about sales and marketing, I would certainly recommend you do a lot of reading and maybe take some relevant courses before you start your business. In truth, there are plenty of businesses, of all descriptions, that offer mediocre products or services - but are highly successful because they are really good at sales and marketing and they know how to run their operations efficiently.
If your town has 30,000 households, then in any given year probably not more than 10% will instruct a professional carpet cleaner. That's about 3000 cleans a year, or perhaps more realistically 1500-3000 per year. If the average ticket price for a clean is about £75 (it could be more), your total local domestic carpet cleaning market might be worth £115K - £225k per year. If you add on commercial carpet cleaning and upholstery - I would guestimate that you local market is worth £175k - £350k per year. These are just ballpark figures, but they do demonstrate that you local area might be able to support eight fulltime carpet cleaners.
If you are still unsure about carpet cleaning, there is no reason why you can't do some test marketing before you start. See what sort of response you get from 10,000 leaflets or perhaps classified newspaper ads. When you generate enquiries, just tell them you're fully booked. Bear in mind that all marketing can be a bit hit and miss, you need to run a campaign over a fairly long period to identity patterns and response rates. This is particularly true for leaflets - you might get nothing from your first 1000 and five or ten jobs from your second.
Provided you are proactive and determined, it shouldn't be too difficult to obtain at least five jobs per week in the very early months of your business. Although leaflets are not my favourite form of marketing, they can be a good option for a start-up because initially you will have plenty of time on your hands. If you delivered 10,000 leaflets per month - I would be pretty confident that you could generate 10-30 jobs (provided your leaflet was well designed). I would also try to develop a marketing mix rather than relying on one or two methods; for example networking, directories, classifieds, direct marketing, referral schemes, up-selling, internet etc. It is a sobering fact, but most new business fail within their first few years, I'm sure carpet cleaning is no exception. Many business fail, not because they do not know what they should do, but because they just don't carry out their plans due to procrastination or inefficiency.