Below the chance graph for a Two Dozens Bet is plotted after 18 spins.
When starting with a bankroll of $100 and betting $1 each time.
Note that you need to bet at least $1 on each Dozen, so your total bet is $2.
You can win 18 times and end up at $118, or lose 18 times and end at $64,
but most likely you end somewhere in between $64 and $118.
There is 46% chance that you end below $100 after 18 spins. There is 35% chance that you gain, and 19% that you end up even !