I know that Martin Ayre's book says 5 or 6 but not sure if that sounds right. There were 2? sets of 3 plus Waite's crashed one was replaced and the Mille Miglia car was another. That makes at least 8.
Listing them from the back of the new book by Canning Brown as follows;
TT Rep Ulster U?? 411 - B G Bailey
TT Rep Ulster - B Belleroche
?? Ulster AXK 978, (incorrect reg. - AXK 828 from photos) - H Birkett
TT Rep Ulster - GW 82 - G HR Chaplin & ST Lush
TT Ulster (Rep?) - WA Cuthbert (Belleroche car)
TT Rep Ulster - CH Fish
TT Rep Ulster - I Henderson
TT Rep Ulster - AZ 3314 - D WM Montgomery (previously H Ferguson)
TT Rep Ulster - JR 3668 - D C Parish
TT Rep Ulster - HJ Searle
TT Ulster - GN 8198
Plus the Mille Miglia car driven by Goodacre and then left (& repainted red!) width Enzo Ferrari. This car had (has?) an English registration as reported in one of the magazines of the time. (I have a photo of Goodacre leaning on the car but it is from the side.)
I make that a minimum of 7, 6 different registrations above plus David Howes AUY 496, (which I believe will be at Brooklands next week). I noticed in the PWA7OC members list Chris Gould also has a TT replica.
Also posted onto the Austin 7 forum