How many pads will I need?

Your 'regular flow' will differ from another woman's 'regular flow', so its is difficult to say with absolute certainty, how many pads you will need. The frequency in which you like to change, also determines this mysterious number. Keep in mind, my pads are very thirsty, so will absorb more than the supermarket variety, meaning you will probably use less than you would of a commercial brand.
Also washing and drying your pads quickly, may mean you get another use out of them in the same cycle, so some women may only need fewer. But factoring poor weather, drying them in time is not always a possibility.

Having said that here is a rough guide to get you started. 

I have a light flow - LINERS x 3, LIGHT x 4, MEDIUM x 3

I have a regular flow - LIGHT X 3, MEDIUM X 4, HEAVY X 3  OVERNIGHT X 2   

I have a heavy flow - MEDIUM X 3, HEAVY X 4, LONG HEAVY X 2, HEAVY OVERNIGHT X2

I have a very heavy flow - LONG HEAVY X4, HEAVY OVERNIGHT X 4, EXTRA LONG HEAVY X6

I've just had a baby - HEAVY OVERNIGHT X4  LONG EXTRA HEAVY X 6   

I have some LBL - 7 LINERS/LIGHT

I need something larger - See my Custom Orders page for anything up to 18" long or 4" wide

Many cloth pad newbies, will buy 1 or 2 pads in there favorite fabrics in a few pad sizes to try cloth out. This helps them work out what suits them best.
However the most popular choice are the starter packs or teen starter packs. These packs are better value, are sewn in all the same fabric and give you a good range of sizes to cover most cycles. 
Many women also prefer to use the overnights or long pads for their entire cycle because they like the extra coverage. Other ladies will only need a handful of liners. But all women who use cloth, love it, and stick with it.