No need to apologise for learning mate!
So, I think it's a normal mash when you do a mash and sparge. It's called a full volume mash if you do the entire amount. It really depends on a number of variables as to how much water you need.
Grain will absorb roughly 1 Litre / 1 kg, so if you use 4kg of grain you will need 4L of water just to end up recovering what the grain absorbs.
You also need to know how much you evaporate during the boil, which will help determine how much water you need to start the boil with. This is your 'pre boil volume' and it differs depending on ambient temp, humidity, elevation, size of boil kettle, how vigorous you boil etc.
Then, to work out how much grain you need to start with, you need to know your efficiency. This means, if the recipe is designed for 75% efficiency, and you only get 60%, you will need to increase the grain amount. This increases how much water is absorbed, and thus how much you need to start with.
You also need to know how much is lost to your particular mash tun (if you can't recover some due to false bottoms or what not, and how much is lost to the kettle which you can't recover.
There are a number of other things which you need to factor in, which determine how much water you need in total, this is why I said you don't usually find a recipe that states "start with x litres of water" etc, because every system is different.
I know in my Guten, I will lose around 3L / Hour of boil, and I lose about 3.5L to trub and unrecovered kettle loss, which means if I want to end up with 21L in the fermenter, I start with 27.5L before I boil. If I know that I am using 5kg of grain, I need 5L ontop of the 27 which is around 32L. Therefore, if I start with 32L, I know I will end up with about 21L in the fermenter.
I also know with my system, that there is 6.5L of liquid under the false floor which doesn't make it to the grain, this is called 'recoverable' mash liquid, but does not factor into the water:grist ratio that you often see people talk about. A good rule of thumb is about 3L of water to every kilo of grain. So, I would need 15L for 5kg of grain, plus my 6.5L makes it 21.5L of water for my mash. I round up to 22L, which leaves 10L for sparging.
I hope this made sense for you?
The easiest way to dial in your process over time is to use something like Brewfather of Beersmith, set yourself up an equipment profile, and each time you brew make meticulous notes. Note down how much water you used, when you added some, did you top anything up, how much ended up in the fermenter etc.
The more you brew and note this down, the more you can fine tune the equipment profile, which will get you a more accurate repeatable result. Then, when you see a recipe, you input that into your program and it will scale the ingredients for you.
Say I found a recipe that's for 2.5 Gallons, I can pop it into Beersmith, scale it to my system and it will adjust for my efficiency, my batch size etc and tell me how much grain I need, how much hops, water etc etc.
It can be quite confusing, and will take some time. BIAB does simplify the process a little. If you let me know how big the pot you are using is, we can try and work out some rough numbers to get you started and then you can dial it in as you go?
