According to Kandel (p. 136), in neuromuscular junctions "At the
crest of these [postsynaptic junctional] folds the receptors for
ACh are clustered in a geometric lattice, with a density of about
10,000 receptors per square micrometer." I'm still learning, but I
suspect the number of postsynaptic receptors and presynaptic vesicles
varies a lot with neurotransmitter and junction type.