Fr Christopher Zugger is cited as saying that there are over 300,000 Ruthenian Catholics in the US. I’m not sure of his source, but the 2017 Annuario Pontificio listed the number as 80,795. Those numbers are self reported by the Archeparchy of Pittsburgh and the 3 suffragan eparchies of Passaic, Parma, and Phoenix.
I think the "300,000" figure was total immigration of Greek Catholics from the Ruthenian eparchies in Europe. The number of practicing Ruthenian Byzantine Catholics in the United States is probably close to 35,000 today.
The numbers from Catholic-Hierarchy.org are as follows:
Eparchy------Population---Parishes
Pittsburgh---57,407-------74
Passaic------11,828-------84
Parma---------9,084-------30
Phoenix-------2,706-------19
--------------------------------
Totals---------81,025-----207
That equates to an average of 391 souls / parish.