As much as possible i can boil with 19 liters pot,
So for preboil 15,5 i can ferment 11 liters,
Can I put all this 11 in the keg? No, so there is no how much i want to put, but how much I can put,
As i said last batch i put 4-5 liters in the keg.
Next i hope i can put more because i will lager with my new refrigerator
Brewing beer is actually just cooking.
Just like when you plan your cooking for how many people you are feeding.
It would be silly to just "cook as much as possible", and hope there is enough for everyone.
You need to plan your brewing for how much beer you are going to make.
That is where you have to start, at the end, meaning batch size, or packaging amount.
Then you need to keep detailed records in order to figure out what all of your "losses" are.
Losses are necessary, like when you are building a house, you can't buy every piece of lumber cut exactly to size.
There is going to be some scrap that is unusable, but you need to start out with it.
You have grain absorption, hops absorption (minor unless you are using a kilo), boil off rate, anything left in the kettle, fermenter losses (trub and hop debris).
There is no need to fret over these losses, you just need to figure out what they are and take them into account.
Personally I lose approximately a half of a liter in my chilling line when transferring from kettle to fermenter, that is just the way it is.
I use to use this spreadsheet to help me calculate my water requirements.
I would also recommend you watch this video to understand water requirements better.
Very important to understand too though is that the most important ingredient in making beer is patience.
It takes years to become a good brewer, and the only way to learn is to brew and brew again, just make sure you learn something each time (I still do).