A frog galvanoscope does not measure voltage. Instead it detects electricity that same way that I detect my static charge after walking across a carpet in winter - by a short current flow. The current causes the frog leg to twitch and me to jump from the shock of the spark when I touch a door knob. (The analogy is reasonably close, since Galvani needed a spark gap in the circuit to make the frog leg twitch.)
In general, a galvanoscope measures current; an electrometer measures electric charge or potential.
The frog-leg galvanometer a kind of threshold ammeter. It tells you that a voltage source can provide a current - at least for an instant - greater than the threshold needed to make the frog leg twitch.
The "56,000" factor comes from page 316 of Wilkinson's 1804 Elements of Galvanism, and is a comparison of charge sensitivity, not voltage. Touching a silver and a copper plate, each 0.1 inch square, to the frog leg made it twitch, but it took 20 touches connected to 6 inch diameter copper and zinc plates to make a gold leaf electroscope visibly deflect. The electric potential in both cases is about a volt. The relative sensitivity comes from the calculation: $20\,\pi\,(6/2)^2/0.1^2 = 56,549$.
I can't immediately find a source that clearly states how much current or charge is needed to make a frog leg twitch. Individual frog skeletal muscle fibres can respond to 50 nA at 50 mV for pulses longer than 5 ms, so this might be considered the ultimate sensitivity for a single-muscle-fibre galvanometer.
Similar results have been observed for cockroach legs.