Welcome to the forum.
Mashing with 1L water per lb grain is about as thick as you can go. Most people (including me) would recommend 1.25 - 1.5L per lb for an APA. I only use 1L per lb for EPAs.
No matter what mash thickness you use, you will lose approximately 0.5L for each lb grain to absorption, so if you start with 15 lb grain, and mash with 20L water, you will only end up with about 12.5L wort. (The other 7.5L will remain in the grains).
When you say 23L for the primary, do you mean that your primary fermenter has a capacity of 23L, or that you want to add 23L wort to your primary?
You need to leave at least 4L head space in the primary, and preferably 6L. Otherwise the fermenting beer will overflow the primary, and you will need a blow off tube, and lose some beer in the process.
Finally, before you add the wort to the primary, you need to boil for about 1 hour. The boil typically boils off about 4 - 5L water, so you will want to start the boil with 4 - 5L more wort than will end up in the primary.
-a.