Limit to Number of Times You Can Get CO2 Laser Resurfacing?

Each CO2 resurfacing thins the skin so after 2 CO2 resurfacings I would worry about thinning the skin further and delayed healing-I would not do it.
Scar removal requires the production of new collagen deep in the dermis to plump and remodel facial scars. CO2 resurfacing just planes down the surface-like sandpaper on wood.
The new Non-ablative fractioanl Erbium lasers, used properly, can stimulate the growth of new collagen deep in the skin and remodel and plump scars.
These newer Non-ablative technques are the choice now for scar removal and are much safer-see article below.

Co2 lasers are great for resurfacing skin or remodeling scars. The laser that you mention (The Sharplan ) has been a very effective and reliable device, we have one we still use on occasion in our office. newer CO2 lasers, however, offer safer more effective treatments usually with less healing time.
For certain types of scars, including surgical scars, burn scars, traumatic scars or acne scars, the Lumenis Ultrapulse CO2 is now the gold standard. This laser can be used as a fully ablative CO2 or used in a fractionated mode (Total FX) to provide safer treatments with less healing time. I often use the Fractionated laser on acne scars or surgical scars because it has an amazing ability to remodel skin [blur scars, fill in depressed scars and sometimes even helps to reduce the lighter color some scars have].
So, to answer your question, it is very likely that your skin would respond well to the fractionated CO2 lasers with less concern for any negative effect from your previous laser treatments. Definitely look into this possibility.
Good Luck

I reviewed your photos and history. In its day the sharplan carbon dioxide laser was a good tool but nowadays better tools exist for the type of scars that you have. Most experienced laser practicioners would agree that a fractional device will give you the best results for the type of scars that you have. The question of whether an ablative fractional device like a carbon dioxide or erbium laser or a non-ablative laser treatment is better for these type of scars is debatable.