>How many people in the US have dysferlin deficiency?

There isn't much in the way of reliable data (at least in the US)
partly because there isn't any central place to collect it.
Statistics collected on the incidence of LGMD as a whole in other
countries vary between 10 and 30 cases per million people. Estimates
of the proportion of dysferlin deficiency in LGMD vary also, but seem
to average about 10%. There's also the question of what proportion of
dysferlin-deficient patients have limb-girdle symptoms, and what
proportion have distal (Miyoshi) symptoms. There was a paper from
France a couple years ago which reported 23 patients with Miyoshi,
which (assuming the same incidence) would imply about 130 patients in
the US (if indeed every patient in France was counted).

Making a guess based on these various estimates, I'd say about 300-600
people...I don't have much to back up these numbers though.

Brad