A few things I learned the hard way in queen-breeding:
Then choose only the largest cells – destroy the rest. And start 50 nucs because some won’t mate successfully.
Numbers will vary depending on conditions, but you get the idea – there will be losses at every step in the process.
Secondly, graft a few more cell cups 3 or 4 days after the main graft. Check the nucs the day after the first grafts should have emerged (a nice, round opening at the bottom of the cell), and put a second ripe cell in the nucs that look like the first didn’t emerge successfully.
Don’t leave them out of the hive very long, as they need to be well-fed. Best to limit the number of queen cells in each cell-builder colony so the queens will develop to their full potential with plenty of royal jelly.