You can do either way....put that caps down and bury them or lay the substrate then push them in. Quantity would depend on how many/where you are putting rooted plants that would use substrate nutrients, but typically you would place them every 3 inches or so. You might want to put an extra one next to each heavy root feeder, like swords.
when i first did my 75g i scattered a bunch on the bottom then lightly sifted sand on top of the pellets once i had enough sand to hold them in place i dumped the rest of my black diamond blasting sand on top shaped the sand, put driftwood in then filled full of water and only found a few pellets that came to the top which i pushed back down, i will be adding 00 O+ caps every 4-6 months after the plants start to take in the next week or two,
I've always pushed them into sand after tank is full and planted.. I think next time I'd put them down first then bury with sand ontop. Technically 1 root tab per every 3" square inches of substrate to be planted with root feeders.