I had a stack of 2 dozen flats which usually lasted about 2 days, 6 infant prefolds and 3 - yes just 3 - size one pastel Kissaluvs.
I used the prefolds under her bum in the sling with a piece of wool between the pf and the sling itself. She very rarely wet the sling though.
The flats were under her bum when she was lying down or sitting on our laps or asleep also with a wool pad under them.
For us, once we had moved away from the idea of a baby being wrapped in wetness we didn't really think about buying many fitteds.
The kissies lasted months because I used them as quick release little pants when we were out and about.
I would buy less than you think you might need and see how you do with washing and drying. Finding nappies in a hurry isn't a problem
