Can we say "He didn't change very many" instead of " He didn't change very much"?
"Many" describes a count of things, whereas "much" describes a mass quantity.
The following dialog is correct, but terse:
Question: He was supposed to change the lightbulbs. How many did he change?
Answer: He didn't change very many.
Whereas, a person's personality is a single thing. It can change a little, or it can change a lot. So it is okay to say "He didn't change very much." But unless the person suffers from "multiple personality disorder", it is not okay to say "He didn't change very many" when talking about a person's personality.