I would check out Baby Beehinds fitteds. I think 24 would be probably the max you would need, if you washed every 2nd day and dried them in the dryer. Both DS and DD wear those as they are OSFA - but you will need covers for them, and the covers are sized.

Best part is - you can wear them at night too - so that stops the costly night nappies

Other than that - you could go Itti Bittis like FJ said - DS is 12.5kg but has a really slim waist and hips (all the weight must be in his big head) and he fits into medium Ittis - DD is probably around 5.5kg now and she fits into the mediums too.