Suppose I have a em wave train.How do I know how many photon does it contain?And where are the photons?(like in the middle or more toward the end of the wave train)The frequency of light that we get from bulbs or sun can be measured so we can accurately state its energy .In this case where does the uncertainty in time ?Moreover when it hits the detector we know about its position so how it causes the uncertainty in momentum?
How do I know how many photon does it contain?
The answer could be very different depending on the source of light. Unless you have a source of pure-state light, like single-photon emitter, the question, IMHO, makes no sense.
You can ask how many photons on average do you have, i.e. what is the expectation value of the photon number operator.