To give a rough idea, according to my calculations it takes 153 inputs (individual action potentials) from CA3 pyramidal neurons to produce a depolarisation above threshold in the CA1 pyramidal neuron. This is of course a very simplistic division that does not consider spatial and temporal summation or any other complicating factor. Also note that each CA3 pyramidal neuron forms a variable number of synapses onto each CA1 pyramidal neuron (~5? according to the hippocampus book), and for each action potential only some of these synapses release a vesicle. Thus 153 inputs = xxx number of activated synapse (activated synapse = synapse receiving action potential).
So how did I arrive at 153?
According to this paper , a single action potential from a CA3 neuron produces an average depolarisation of 131 uV, presumably at the cell body, which we can also take, for simplicity, to be the depolarisation produced at the axon hillock. So assuming the threshold is 20mV (I'm not sure exactly but it should be in the 20-30mV range), the number of CA3 neuron each producing a depolarisation of 131 uV required to reach threshold is 20/0.131=153.
Again this is a simplistic division and may not be a good estimate once you take into account all the complicating factors in summation, but hopefully it gives some idea. Also bear in mind that synapses in different parts of the brain can have very different properties, so 153 might not be applicable to them.
 The time course and amplitude of EPSPs evoked at synapses between pairs of CA3/CA1 neurons in the hippocampal slice.
RJ Sayer, MJ Friedlander and SJ Redman.
Journal of Neuroscience 1 March 1990, 10 (3) 826-836