This topic has been discussed ad naseum on betting sites and forums for ages and my advice is to keep it really simple.
Divide whatever your bankroll is by 100, whatever amount you get will equal 1 unit. In other words 1 unit is 1% of your bankroll. The reason why sports bettors tend to write their bet sizes in units instead of dollar amount is because it shows how big of a bet it is relative to their bankroll. If I were to say I bet $500 on X you would have no idea if $500 is massive bet for me or just a standard 1% of my bankroll bet.
For example if you have a $1000 roll, 1 unit will be $10 for you($1000/100).
I suggest betting anything between 0.1 unit to 5 units per bet. 5 units is the ceiling, only bet 5 units when you’re very confident. So even if it goes to shit you will still survive and not worry about busting your roll.
The typical bet should be around 1-2 units. Your high risk high reward bets should be less than 1 unit and your “confident” bets should be 2-5 units. Adjust accordingly. But please try not to ever break the 5 unit ceiling. No matter how confident you are it’s very important that you don’t put a large fraction of your bankroll at risk in one single bet.
If this is the first time you’re reading about bankroll management you will probably be surprised that the typical bet is only a few percent of your bankroll at most. Even if you’re a winning sports bettor you will eventually run into downswings and if you play a more aggressive bankroll strategy it’s a question of when not if you will bust. I don’t care how good you are at this nobody has a perfect run, the bet sizing is to keep you from busting when you have a bad month.