Well, I only use one, (at a time), but you'd need to allocate 2 (or more) so as when one's being washed, there's another one to go in. I can get away with one because I have extra pee pads (that I made myself) strategically placed in places that get peed on the most, so as it's got extra absorbency. I think people who just have fleece and towel without any extra pads use two at a time generally.
Your fleece needs to be 100% polyester. I use argos fleece blankets, but any polyester fleece will do. It needs washing 3-4 times before putting in the cage (and allowed to dry in between washes), with detergent only, no fabric softener or dryer sheets etc. You can also put 1 cup of vinegar in, some people say it speeds it up, I've never done it personally. After the few washes, just pour a little bit of water on to the dry fleece, with kitchen towel or something underneath. If it soaks right through to the towel, it's wicking the moisture. If it takes any longer than about 10 seconds, it needs washing more. Basically you have to break down the fibres, so as any urine will go right through the fleece on to your towels, leaving the fleece dry (and more importantly hygienic) for the pigs to sit on.