The country supplies some two-thirds

The country supplies some two-thirds of anti-retroviral drugs globally to fight HIV. (Cough syrup deaths: Why drugs made in India are sparking safety concerns by Soutik Biswas)

Does "some" mean "about" or "some of"?

It's the same as "about" there. (It's a common usage.)

It's a way of implying that two-thirds is a considerably large proportion.