Every Newbie's Fantasy
Covering large expenses means that only a player with EXTREME skills can even attempt this.
The real problem is that a talented Card Counter has an overall edge that varies, but realistically speaking, tends to be near 1% of all monies wagered.
"Risk of Ruin" is not an issue as you are positing, by implication, a bottomless bankroll.
However, and here is the rub - The more you bet the tougher the game becomes as the casinos will strive ever harder to prevent you from profiting.
So, to answer your question in a pragmatic sense, not in terms of mathematics, there is no "size of bankroll" sufficient.
As the casino's manifest opposition to your attempts to make money is directly proportional to the stakes that you play.
To draw an analogy, as one gets closer to the speed of light time slows to zero and mass approaches infinity. (I think that I got that correct) L.O.L.
So ... in conclusion, reifying your fantasy is virtually impossible; although I suppose if you travel in steerage, live on stale bread, sleep under bridges, and accept a high probability that you will simply fail to win anything, then perhaps $50,000 will suffice --- presuming that you are VERY skilled of course..