Okay, then he gets a random number and it ahs to be below 49.5. I assume the maximun number is 100...
The range of numbers you can roll are from 0.0000 to 99.9999; that is exactly a million different numbers.
But still, I don't understand how much money he bet, and how much money can you make if you win with this type of win (I guess it depends on what number you choose, and can you even choose the number? I mean the <49.5 number).
You can pick your chance of winning, and whether you want to bet high or low.
In this case he picked a 49.5% chance of winning, and bet "low".
If he had bet "high", he would have had to roll 50.5 or higher to win.
The payout multiplier is always 99/chance
In this case the chance is 49.5%, so the payout is 99/49.5 = 2 - ie. you double your money when you win.
You can choose a chance of winning as low at 0.0001%, which means you have to roll lower than 0.0001, ie. you have to roll exactly 0.0000
That happens around once per million rolls, and pays 99/0.0001 = 990,000 times your bet amount.
You bet size is limited such that your potential profit per bet doesn't exceed the site limit. The limit adjusts dynamically based on the size of the site's bankroll., and is currently 152
BTC. That means that you can bet up to 152
BTC at 49.5%, or up to around 0.000153
BTC at 0.0001%.