I'm posting anonymously because my employer has instruments on the rovers.
For the rovers and part on them, about 100 prototyping models were built (that is, 100 rovers which never got off the ground, and 100 models of every device on them).
Flight test and spares are another 20 items. Making more rovers does not require building more test models - and as has been proven with Hubble - ground test spares can be flown if need be.
There are enough flight test spares to trivially launch 2 more rovers. Personally I'd push for better tyres now that we know the composition of the surface and some form of "dust brush" for the panels as well as better "cold rating" for the components given that we know the design can survive several seasons.
HOWEVER: the rovers are quite limited in what they can do. Flying 20 more would be a good thing - but so would flying 20 more Beagles (my own estimate after reviewing all the documentation and talking to people involved is that if enough funding was made to fly 12 then only 1 (possbly zero) would have failed - Beagle suffered from chronic funds shortages which led to catastrophic decisons to reuse parachutes/airbags which were repaired prototype testers and which likely simply froze solid en-route due to leftover moisture (even after operators of the UK's largest vacuum chamber spent 6-8 weeks doing their absolute best to get moisture out of the airbags they were still seeing traces when time ran out - and the attempt destroyed a number of very expensive pumps never intended to cope with the amount of water involved))
The proposed larger missions are _much_ larger physically than the rovers - carry a lot more instrumentation and have much bigger/more powerful wheels/treads (and solar panels) than spirit/opportunity.
It would be nice to see all the missions fly. That's unlikely to happen in any of our lifetimes unless payload costs drop by 99%. Until then what goes is based on who has money and the political will to send things - space exploration is a strange, political universe which attaches far more importance to the flag-waving aspects of launches, manned programs and high profile results than to funding the ongoing science required to actually support these activities both after the rocket's disappeared from the ground and when things are in planning stages.
"We" (worldwide) are in grave danger of losing vast amounts of irreplaceable space mission data because no entity is willing to fund storage or distribution. In a decade I hope that isn't being said about data from these incredible wee rovers.