The OP asks

Why did the authors of nginx choose eight, and not a higher number? What could go wrong if I add more buffers, or a bigger buffer size?

This was answered by one of the authors in the nginx forum

Some hints about seveal big vs. many small buffers:

• Using bigger buffers may help a bit with CPU usage, but usually it's not a big deal.
• Using bigger buffers means less disk seeks if reply won't fit into memory and will be buffered to disk.
• On the other hand, small buffers allow better granularity (and less memory wasted for last partially filled buffer).

Since there are different load patterns (and different hardware) it's hard to recommend some particular setting, but the about hints should allow you to tune it for your system appropriately.

In general, you should probably search the forum first if you’re looking for an answer from the nginx developers.