I have had 3 c-sections so far, and am having my 4th in a few weeks. I was unable to have v-bacs since my 1st and 2nd child were less than 2 years apart, and the doctor would not allow a trial of labour. (We had also moved from another province when I was about 8 months pregnant, so I did not have time to look for a 2nd opinion). After 2 c-sections, and then a 3rd baby less than 2 years after the 2nd, my husband & I decided that the risks of trying a vbac were too high, and any future babies would be delivered by c-section.
I think the number of c-sections allowable should really be decided on an individual basis. Some women are fine to have multiple c-sections (I know a lady who had 7), while others whose scars have not healed as well may be taking larger risks in having many. Unfortunately, a lot of doctors are worried about getting sued, and so will not do more than 2 c-sections.