0.1 0.2 0.4 0.5 0.6 Net number of units profit Graph shows effect of different win percentages over different numbers of bets. Professional bettors tend to have a lot of bets compared to non-professionals. More importantly, a bettor is more assured of achieving his expected win percentage over a larger total number of bets, and with more bets and smaller bet sizes, the 'ride' is much smoother, less risky, and more predictable. Generally speaking, non-professional gamblers go wrong by risking too much of their bankroll on individual bets. They don't spread their risk over enough bets.
Professionals use smaller bet sizes in proportion to their bankroll over larger numbers of bets.
If you're a part-timer with a wad of 'mad' money to risk, it may be okay to risk 4 or 5 percent of your bankroll on a single bet, but don't expect to play that way for long. Sooner or later, such mismanagement will annihilate you. To quote from my book, How Professional Gamblers Beat The Pro Football Pointspread, the population of bad handicappers in Tap City is dwarfed by the population of bad money managers. Of those genuine professional-level sports bettors I've known, most would never risk more than two percent of their working bankroll on any single bet, and all of them usually risk much less than that. The general consensus seems to be about one percent.