how much money should I lose?

[Farm Yard Forex]

With good risk management you can hopefully avoid total loss but if planning for that worst case scenario then look at whatever you can afford to lose that won't mean rent not being paid etc. It should be spare money.

 

[Jungletrader]

Aim to only loss 1% per trade should give you breathing space .
Loses are part of doing business once your monthly stats are BE or slightly in DD I wouldn't stress too much