there already is a limit it is the relationship between adoption (what you say number of wallets), price rise, difficulty, profitability of mining and the number of miners.
if there is low adoption, the price will be low and difficulty remains low with the low number of miners.
when adoption increases, price goes up which make mining profitable so more miners join and difficulty goes up.

bitcoin has 0 carbon footprint because it is a network and bitcoin nodes, wallets, and bitcoin miners don't produce anything at all.

the electric companies on the other hand that are producing electricity have a terrible carbon footprint. so maybe you are asking the question in a wrong place!