Ooh Annat! Where did you buy from???
I would say, umm...one booster per pocket nappy? There are all sorts of boosters you could get.
(Quick lesson: hemp and bamboo are very absorbent. Microfibre is very absorbent, AND it absorbs quicker than the others. It *can* get stinky over time though. Nothing a bit of fresh air doesn't fix. So for Riv, who wees all at once, I would be putting microfibre closest to his butt, to quickly soak up the wee, and then hemp or bamboo underneath)
Some people even get microfibre cloth from the supermarket (think it's in the car aisle...think shammys are a type of microfibre?).

ETA: lol Chrissy! Flats are fine! Just by the time I've got the pin done up, I could fit my head in all the gaps round his legs...I was too scared of pr!cking him as a newborn and then I just got used to the other nappies so I haven't tried a flat in ages!