Vlk
4
Approximately how many viruses does Avast recognize according to it's virus definitions which I gather are constantly being updated????
All of 'em. 
And the updates are necessary because new viruses appear (almost) every day.
Anyway it’s pointless to say “how many” as there are no ‘naming standards’ – for a more detailed discussion, please see e.g. this thread: http://www.avast.com/forum/index.php?board=2;action=display;threadid=200;start=0
Hope this helps,
Vlk