Everyone has to figure out what works best for them. My own preference is to get feedback from 2 or 3 beta readers, but I will also look for comments on just the opening chapters.

I also find it very useful to critique others, both to pay back/forward favors, and to hone my editing skills.

The key for me with beta readers is to find people who get what I'm writing, but who are good at pointing out where I can makeep my story stronger.