What is the collective noun for envelopes?

There isn't a widely accepted or commonly used collective noun specifically for envelopes.

You could use a general term like "collection of envelopes", "set of envelopes", "batch of envelopes", or "stack of envelopes" depending on the context.

If you're feeling creative, you could use a more descriptive phrase like "nest of envelopes" if they are stacked together, or "flutter of envelopes" if they are scattered around.