In order to make Max Soup at the Master Dojo on the Isle of Armor, a player will need three Max Mushrooms. These special mushrooms are pretty rare on the Isle of Armor, only growing in dark and moist locations. On the Isle of Armor, there are only two places like this: Forest of Focus and Warm Up Tunnel. When you first find Max Mushrooms during your second Trial given to you by Mustard, you will find a cluster of three Max Mushrooms at once. However, after this occasion, Max Mushrooms will usually only be found once at a time.
Max Mushrooms will gradually respawn in the same locations, but they will do so at a pretty slow rate. But there is a way to make them grow back faster, and that method lies solely in Max Raid Battles. You see, by defeating Max Raid Battles across the Isle of Armor (and only the Isle of Armor), the Max Mushrooms will grow back quicker. It appears that a Max Mushroom will reappear after three different Max Raid Battles have been completed. This means that by defeating nine different Max Raid Battles on the Isle of Armor will see players harvest three Max Mushrooms - the exact number needed to make Max Soup!
So, if you're looking find and obtain every Gigantamax Form, it appears you will need to participate in loads of Max Raid Battles. Not only will you have the chance at finding Gigantamax Pokemon in Max Raid Battles, but you will also be able to stock up on Max Mushrooms so you can make Max Soup to obtain the same! There are exactly thirty-one different Gigantamax Forms to obtain now, which means only ninety-three Max Raid Battles (or less) will need to be completed in order to obtain each and every last one of them.