From Game Revolution: “Given how many Nazis there are to shoot, there is kind of a Wolfenstein Youngblood ammo storage problem. You murder a few Hitler-loving jerks and before you know it, youre out of bullets. So how do you upgrade the Wolfenstein Youngblood ammo capacity so you can increase how much ammo you can hold? Thankfully, its not too hard. Its like if the game wants to make it easy to kill as many Nazis as possible.”
Source: N4G PC Wolfenstein Youngblood Ammo Storage | How to increase and upgrade it