Essentially the answer to your OP question is infinite. You can get as many reds/greens as the universe can give on any betting streak regardless of whether it is martingale or not. This is called luck and you cannot use martingale to evade your bad luck. If there was such a method then casinos would have gone bankrupt a long time ago
They don't go bankrupt for two reasons
And none of these reasons has anything to do with the requirement of an infinite bankroll of the gambler. The first reason is that casinos limit the amount you can win (yeah, regardless of whether it is martingale or not). And the second reason is that only a small (mire like tiny) fraction of gamblers are using martingale in the way it could and would ruin a casino in the blink of an eye if there were no restrictions
To keep it simple, just imagine there is only one player with a huge bankroll and a small house edge and a number of player with small bankrolls but compared to the first player if combined. In this case, you could make it even simpler and reduce to just two players with equal bankrolls, one of which is using martingale against the other to overcome the house edge of the latter. Who will bust the other player sooner?