xboard -mg 10 -lpf STARTPOS.fenwould play a match of 10 games from the start position in the file. If there are more positions in the file you can specify you want to play from number N with
-lpi N. If you specify
-lpi -1 it would start at position 1, and play every game from a new position. With
-lpi -2 you would play every two games from a new position. If you add
-rewindIndex M you would start at game 1 after having used the first M positions. (For if you ask for more games than you have positions in the file.)
The same works if you want to play from openings given as PGN; in that case you have to use
xboard -mg 100 -lgf OPENING.pgn -lgi -2 -rewindIndex 10to play the first 10 openings of the file 10 times (in 5 pairs of white and black).
Of course you would have to specify engines as well, if you don't want Fairy-Max to play itself.
Plus things like time control and other parameters, if you don't use XBoard 4.20100118, which loads them from the settings file.