Initial print runs tend to run at about 5000 for non-fiction books. New technology means that publishers can take a suck it and see approach. If the first batch sells then they print more but I've also heard that some are printing as little as 2000 nowadays. If you find a publisher you will usually get an advance equivalent to the sales they expect in the first year. It basically depends on how well they think the book will sell.
Lots of publishers are also embracing new technology more and publishing for kindle.
I've ordered initial print-runs for non-fiction books, and they vary a lot. Much depends on the book itself, as genre, author, and potential market each affect this; then you have to factor in the territory, and the rights you have within that territory, and so on.
Consequently it's difficult to say what an average first print run is; and I'm not sure it's useful, either, if we're talking to individual writers who are interested in finding out about their own books.