I don't know if you've already consulted the Wikipedia article here:
https://en.wikipedia.org/wiki/Fiat_126 and this item too:
http://www.gizmohighway.com/autos/fiat_126_history.htmWikipedia gives a lot of information, and especially if you click through to reference 1 you'll find others have been there before you. You might also try posting on the general Fiat forum and the Fiat 500 forum, see what the folks there come back with.
It's a popular pastime, to try to create "definitive" lists like this, and I wish you well, don't get me wrong, but the motor world is so complex and fast-moving that even manufacturer records are typically incomplete and have errors. The 126 was produced in millions, over 3 million in Poland but only a million in Italy (so is it in fact a Polish car rather than Italian ... ? [discuss]) with only a couple of thousand in Austria and who knows how many by Zastava in Yugoslavia.
So your first task will be to define your region - cars imported into the UK only, or *all* cars produced? And then what about cars made in, say Italy, but with their own specification for the German market (there may be other examples but the German one I know is documented)?
Then there's the Seat 133 made in Spain, is that a 126? FSO made an estate version, and the Bis was apparently never actually made by Fiat (is that true?)
And do you include also the after-market customisers who did convertible and beach buggy style conversions?
It's a Herculean challenge, and a great basis for pub quiz experts to come out of the woodwork and say "ah yes, but I think you've overlooked ...."
Good luck!