Why is it important to have more than one piece of evidence to support your claim

It makes your claim valid.

Explanation:

The more evidence of something, the more realistic and valid it may seem. Let's use a crime scene for example. If you're an investigator and believe someone did something due to only one piece of evidence, there's a low chance of it being either believable or correct.

I hope this is a good explaination, i'm not the best, but either way good luck and have a good day.