Does it require a large amount of water to cause infection from naegleria fowleri?

These parasites are very small so many can be present in a small amount of contaminated water. However, they are not common in the surrounding water people are used to expose to such as rain water, household water or river water, etc. so you should not concern too much about this deadly but very rare parasite. Good luck.

The amoeba is present in swampy brackish water and typically is not found in tap water, puddles from rain, or water in well-maintained wells.