All projectiles from all sources (Volley, LMP, GMP, items, skills, etc.) are simply added together. Then they are distributed according to this pattern (courtesy of Mark_GGG):
For example, with Volley, legacy Reach, and Endless Munitions (Deadeye ascendancy) you will get:
1 projectile base,
1 extra projectile from the passive,
2 extra projectiles from Volley,
4 extra projectiles from the bow,
for 8 projectiles total.