the answer in catering is always
time of day you are serving
number of hours you are serving
demographics of the event and guests
e.g. certain groups - like football players would eat more than say models.
but it varies from culture to culture and area
certain groups are known large eaters and others just pick and drink.
also depends on the actual items you are planning to serve (and time of day)
for example if you are serving hors d'oeuvres in the late afternoon its more of a snack. but during the dinner hour it really does become dinner and so you not only need more food but bulkier food that will fill guests up.
there are some vague rules of thumbs you will see tossed out
like 5 pieces per person per hour but again that's subject to all the other criteria (passed vs. stationary, demographics timing etc) I would say in a normal 2-3 hour party 15-20 "bites" per guest is usually more than enough but "it depends"
you might also want to post this in the catering forum.
and it is helpful to post the actual items you are thinking of serving.
each caterer works differently. Some do equal amounts of each item
eg 1 of each item per guest
or some like to base it on their feeling of popularity (and cost) of each item and may only have "1/2 per person of item A" eg for your 50 guests only 25 but have 2 -3 pieces per person of a more popular item.
it also depends whether you are passing the hors d'oeuvres,
or putting them out in a stationary display (requires more product usually to keep displays looking full)
not sure if this answers your question but as I said more information from you (see some of my criteria that I would ask my clients
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