Placement depends on the tank footprint and plants.
A good rule of thumb for moderate root feeders 1 gel cap every 1/2 foot. So a 40 breeder would take about 8. They last at least 3 months sometimes 6 months.
You can buy a bag of a 1000 size 00 gel caps for $10 on eBay.
Not worth it to make it IMO when you can buy a 100 for $6 also on eBay that will last you years in the tank mentioned above. Even a small bottle of Osmocote will make roughly a 1000 root tabs.
Full disclose I make and sell them. If you are thinking about selling the one you don’t want the profit margin is very little say less than $.50 for every 100. I buy in bulk and still only make $.75 for each sale that’s after 10-15 to make each 100.
I don't spread them throughout my tanks. I put them below the root feeding plants that need them. Usually 3-4 underneath big and heavy root feeders, like Amazon swords, and 1 below other plants (or 1-3 below a close group of plants).
I add more every few months, or whenever I notice that the plant might need them.