This is not a question that can be answered with a simple number. It will depend upon the characteristics of the AC that you're using. I have some experience with AC mixed with fly ash being used as a concrete additive, and there can be a wide variability in how AC affects foam index, dependent upon the pore size distribution of the carbon. More microporous AC (that made from coconut, anthracite, bituminous and sub-bituminous coal) will have much less impact on the foam index than mesoporous carbon (made from lignite, peat, or wood).  You should first determine what the raw material of the carbon is, then perhaps investigate its impact on foam index.

ASTM has a good report series on the different foam index tests (there is no industry standard) and you may be able to test your limits with testing before experimenting with AC as an additive. http://www.astm.org/DIGITAL_LIBRARY/JOURNALS/JAI/PAGES/JAI101601.htm

Thank you Martin for your answer!!! and I understand this is not a simple number answer, but your comments give me some light as how to do the first attempts.

I will be trying with AC from coconut shells, do some lab experiments, and then have some conclusions to be done. I will let you know how it works.

Thanks again!!

This depends on the type and the particle size distribution of active carbon you will use...

I have a related question, forgive the intrusion, but it appears there may be someone here who knows the answer to a question I posted elsewhere.

https://www.researchgate.net/post/If_I_placed_granular_activated_charcoal_GAC_in_a_container_and_purged_it_with_CO2_gas_would_the_CO2_displace_oxygen_that_was_adsorbed_on_the_GAC