Do you mean P1? I always kept my money to a minimum, because the more you got on your bank account, the more fighter appeared in random fights against you. Just buy expansive cargo which you sell when you need to buy new missiles etc., and you`re doing fine
Back to topic, I never had the patience to gather more than 2 million credits. Whenever I get close to that number, I get bored and just restart the game.
O.K. Looks like 12 million credits is the most anyone can come up with. I can get 10 million using BANKROB.EXE, I've legitimately built up 5 million or so, so the amount I am going to use is 15 million credits.