What is the number of synaptic vesicles in an axon terminal?
Short answer
Several to thousands of vesicles per synapse. In the cortex, 0-1 are released per typical synapse. At neuromuscular junctions or specialized CNS synapses there can be more.
Longer answer
It doesn't make much sense to talk about an &quot;average&quot;, since there are different types of synapses with very different properties, but we can talk about some examples from synapses used as models. In Rosenmund &amp; Stevens, 1996 they estimate about 10 readily releasable vesicles at one synapse in hippocampus. Rizzoli &amp; Betz, 2004 count hundreds at a frog neuromuscular junction.
In hippocampus and neocortex, for a typical synapse there are from 0-1 vesicles released per spike. Higher release probabilities to activation of single cells are achieved by making multiple synaptic contacts; in a typical cortical synapse, there's really just one active site so a max of one vesicle released (Silver et al 2003; Auger &amp; Marty 2000 and Abbott &amp; Regehr, 2004 also discuss this a bit).
Short-term plasticity can strongly influence release probability (Abbott and Regehr 2004 is a good reference to thinking about synaptic computations like this). Multiple action potentials occurring quickly can cause a sustained increase in calcium concentration in the presynaptic cell, increasing release probability at otherwise low-probability synapses (short-term facilitation). Alternatively, many spikes over a longer time period or at higher probability synapses can exhaust the pool (short-term depression). Rosenmund &amp; Stevens used this to estimate the readily releasable pool size of about 10: they triggered a ton of rapid-fire spikes and counted release events, finding only 10. Rizzoli &amp; Betz mention that 10 seconds of 30 Hz stimulation (300 pulses) depleted the available vesicle pool.

Abbott, L. F., &amp; Regehr, W. G. (2004). Synaptic computation. Nature, 431(7010), 796-803.
Auger, C., &amp; Marty, A. (2000). Quantal currents at single‐site central synapses. The Journal of physiology, 526(1), 3-11.
Reyes, A., Lujan, R., Rozov, A., Burnashev, N., Somogyi, P., &amp; Sakmann, B. (1998). Target-cell-specific facilitation and depression in neocortical circuits. Nature neuroscience, 1(4), 279-285.
Rizzoli, S. O., &amp; Betz, W. J. (2004). The structural organization of the readily releasable pool of synaptic vesicles. Science, 303(5666), 2037-2039.
Rosenmund, C., &amp; Stevens, C. F. (1996). Definition of the readily releasable pool of vesicles at hippocampal synapses. Neuron, 16(6), 1197-1207.
Silver, R. A., Lübke, J., Sakmann, B., &amp; Feldmeyer, D. (2003). High-probability uniquantal transmission at excitatory synapses in barrel cortex. Science, 302(5652), 1981-1984.