Approximately how many viruses does Avast recognize according to it's virus definitions which I gather are constantly being updated????

All of 'em. :wink:
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