I believe it is more about the location of the lesions. My MRI reports never had a final number either--last count was 40+ with several over 1 cm. At that point my episodes were very tame--I usually just passed out for a short while but was fine within a day or two. My last episode was in Mar 2011 and it was a large lesion on the brain stem. Life has pretty much sucked since then. I was on Capaxone but I switched to Tysabri after the last episode.
I have heard of autopsies on people that had lived long, healthy lives that found numerous brain lesions when the member was never known to have any symptoms and was never diagnosed with a disease. I think that reinforces the idea that it is the location of the lesions, not amount or size.