Using the auto-complete function is for me about improving conversational and thought-process flow, reducing the barriers to response.
I find the search subset is not too much larger than what I can reduce it to by (as you mentioned) “typing more characters”… hence @tobiaseigen’s request for “10” results.
There are ~21 people with “smith” somehow in there name or username here on meta.
I wouldn’t expect the ability to scan though hundreds of results.
If using ↓ to move down though the list then displayed “infinite list” scrolling style spinner loading 3 or so pages of 8 items that would make me happy.
The likelihood is that it won’t get to three pages of results within the search results.
Should it load more than 3, I don’t know - is there a problem if it does?
I find auto-complete to also be an issue on mobile in this way, just can’t quite remember that additional character that will get you to the username you need. You can’t just open a new tab, search and return as easily on mobile - you can and I do - but it sucks.