That's a good question with no good answer.
> a driver is spec'd by an ASIC vendor as a 12ma, 6ma, etc what does
> that mean
Pretty much whatever the vendor wants it to. In some cases the
number is more-or-less worst case, sometimes typical, sometimes
at useful voltages, sometimes not. At VLSI in our 350nm library
the Iol is measured at near-worst-case and 350mv, while the
Ioh is measured at 2450mv. Notice that the pullups are weaker
than then pulldowns. In our 200nm libraries the numbers are at
350mv from the rail for both pullup and pulldown.
> as far as its expected VI curve
Anybody's guess. Process variables, predriver supplies,
possibly architecture, all influence the shape of the V/I
curve. About all you can count on is the slope near the rail.
> and how many loads
> I can expect it to drive?
Welcome to the world of signal integrity. You aren't driving
the loads, you're driving the wires. If you're willing to wait
long enough there's almost no limit on the number of loads even
a weak driver can manage.
-- D. C. Sessions email@example.com