1. People don't just practice at formal ranges. Some have personal property where they practice. There are also competitions and events of all sorts in which copious ammo is needed.

2. You can actually make your own ammo (usually by using used casings from previous ammo)

3. In times of civil unrest (like the riots of 2020) 50 may be nowhere near enough. In high pressure situations it may take a full clip/magazine to take down a single opponent.

4. I am not a hunter, but I somewhat doubt 50 rounds of ammo would be enough for hunting.

5. The vast majority of people with copious ammunition are law abiding citizens, and so this would involve a confiscation and restriction on the behavior of citizens who have not and will never commit such crimes.

6. Considering how poorly we've done and restricting access to weed and other drugs, I see no reason that any such legislation would actually be effective.