How many drinks do each person drink in cocktail hour?

It depends on the types of drinks you are serving and the crowd. On average it could be 5 drinks but if the crowd is heavy drinking I would calculate it at 8 drinks pp

For a 5-6 hour reception we usually recommend 6 drinks per person. Some people will have 3 or 4 drinks, some people will have 8, some people may only have 1 or none.. so 6 drinks per person is a fairly safe estimate. If you know of some big drinkers that are attending you could up the quantity to 8 drinks per person. Unless they are planning on having a big night, most people will consume 2 drinks in the first hour and then 1 every hour after that.

This is not including water. Water will need to be added on top of this. If supplying bottles of water i would suggest 2 per person. If your water is on tap you don't have to worry about that.

I usually allow half a bottle of wine pp and 2 beers pp overall ... eg. 100 guests (50 bottles of wine and 200 beers) would be sufficient for a 4 hour service

If you are having cocktails initially (for approximately 1 hour), I would allow 1 cocktail pp overall before guests will move over 2 beer and wine options

On average six

I think this really depends on the type of cocktails you are serving & the age's of your guests. If you have a younger guest list & you are offering fruity/light cocktails then I would say around the 2 pp per hour (possibly 3 if it is a hot day). However if you are offering espresso martinis, I would suggest around 1 per hour, & if you have an older guest list I would suggest around 1 per hour.