There is no max level, you only stop leveling up once the required xp to level up surpasses 999999999 as you cannot get more than this amount of xp. This is due to the game not being able to load a number over 999999999.

Best place to farm Exp is at your second visit to Hirata Estate just before the Owl boss fight. Rest at the idol, destroy all the paper walls and obstacles then kill the black shinobi hunter, kill the white one from behind and then the 2 archers in front of him. You can use shuriken to destroy the walls, wait a bit and the enemies go back to their starting positions. Rest at the idol and repeat. The paper walls stay broken. For best results rush to NG+7 and do it without Kuro's Charm and Demon Bell active.

Best farming spot is Ashina Castle - Antechamber idol after the first attack. Stealth kill both blue samurai and purple ninja and repeat. Takes about 20 seconds and it gives you ~3000XP.

cocke n' ball torture

Please,can i spend my xp before a boss fight for exemple?