Most fund managers and professional traders risk no more than 1% of their trading capital on each trade and most of the time a lot less.
If you risk more than 1% on a single trade you risk having large draw-downs in your account but most importantly suffering damage to your trading psychology. The reason that professional traders risk such small amounts has nothing to do with the fact that they expect to lose 20 trades in a row. The simple reason they trade with such a small risk is so that a string of losses even 4 or 5 has no impact on the account but more importantly has no impact on their psychology, so they have no problem taking the next trade.
I have known very good traders go broke because they risked too much in their trading usually because they got too confident or other traders who have suffered 60% draw-downs in their accounts for the same reason.
If you suffer large draw-downs all because you risked too much on the trades the effect it can have are shocking. For example you may find yourself too scared to take the next trade or trying to find another trading system that works better. These things will kill you in trading.
Take away: You must keep your risk per trade to 1% or less.
Next: Stop Losses