In the UK, the CAA approves procedure design organisations - listed here [0]. There are 8 and several are only a couple of people. Even the large ones probably only have 4 people max actual IFP designers. The total involved in the UK probably does not exceed 25. You could scale US according to the number of IFP airports.

[0] https://www.caa.co.uk/data-and-analysis/approved-persons-and...