For example if we will have a massive crash due to regulation and BTC is going to 5-12k how would it effect stable coins?
Those bitcoin holders may dump big chung of bitcoin that will drained liquidity from the market and dumping the price of stable token if there would not be enough liquidity
When you sell a large amount of bitcoin on the market, someone will buy it back, otherwise you cannot sell bitcoin when no one is buying, so this will not affect stable coins. If it happens it will affect the whole market because bitcoin is the biggest coin in the market. Insufficient liquidity only happens with shitcoins or you are trading on an exchange that is not large enough and it cannot affect the entire ecosystem of a stable coin. USDT lost its peg in the past due to competition among stable coins, not due to a bear market.