Average
I don't think there is such a thing as an average number. How many rejections you receive depends on the quality of both the query letter, and the manuscript itself. A bad query letter can stop you cold, and the manuscript itself, however good, will never be requested. A bad opening in the manuscript can stop any agent from reading past page one, etc.
The same bad query will probably be rejected over and over and over and over. But change the query just a bit, turn it into a good query, and the first agent who sees it will likely request the manuscript, assuming you're querying the best agent for the job.
The same bad opening will cause a manuscript to be rejected countless times, but change the opening a bit, turn it into a good opening, and the first agent who reads the manuscript will likely take it on.
Very often, when you hear that a very good, bestselling novel was rejected many times, it really wasn't. A bad query or a bad opening was rejected many times, and after the writer made a few changes to one or the other, the manuscript was snapped up.