Estimating sugar & cream for a coffee service
My company runs several coffee events per month. Here are some of my formulations that are a result of doing coffee tastings and events around Seattle: 

3-oz of coffee by weight, brewed with 62oz water will yield and aiprot or a standard 12-cup pot of coffee (60-oz brewed coffee or 7 servings if you are using 8-oz hot cups). For coffee gigs I bring 1-pound of regular and 3-oz of decaf coffee per 25 guests. So for 50 guests you will need 2-pounds of regular and decaf is never more than 20% (even after dinner). So for 50 guests you will only need a half pound of decaf. And will probably not use more that one pot of decaf coffee.

Cream: 1-pint half &amp; half per 25 guests. We just put the carton of half &amp; half in a small bowl of ice. We use Organic Valley half &amp; half with the little plastic cap/spout. It works well and is one less thing to clean up after the gig. 

Sugar: 5 packets per 25 guests.  

If you are worried about getting hit hard with coffee drinkers just double all of the above.