how much the price can deviate from the price of the StopLoss, if the the StopLoss is reached and it should be executed
The Mechanics of stop losses alongside MT5 settings to it:
Stop losses and take profits are not guaranteed prices your trade will close on. Instead, they are trigger points that, once breached, will trigger a message to be sent to the trading server to close your trade. Once this message reaches the server, the trade will be closed at the best available price on the market at that time. If the price jumps above your stop loss or moves past it quickly, you will receive the next available live price.
Regarding MT5 settings and its extensive features, MetaQuotes (the developer of MT5) best explains them on their website.
Please see the section 'Trading', covering both Deviation and Stop Levels, on the webpage - https://www.metatrader5.com/en/terminal/help/startworking/settings