Here is my note from the beverage planning table on punch (which sangria counts as) for 100:

(1/2 cup punch per serving)
6 to 10 gallons
10 gallons allows 3-4 drinks per person, needed if served alone. Use the smaller number if coffee and tea are also served.
PLUS ice
You would want 1/2 for 50

Puerto Rican Sangria (Sangria Criolla)
This is quite strong; use more wine to reduce the punch of the punch

1 1/2 cups frozen passion fruit pulp, thawed (may use frozen guava concentrate)
2 cups pineapple juice
1/2 cup fresh lemon juice
2 cups red wine
2 cups lemon-lime soda (ginger ale is also good)
1 cup white rum
4 sliced oranges, lemons, tangerines, etc

Combine all of the ingredients except the ice in a pitcher. Chill for at least 1 hour Overnight is better) before serving. You need about 4 pounds of ice for this recipe.

Or for a bit more show:
1 bottle of cheap red wine
4 fl oz of vanilla liquor
2 fl oz of brandy
2 fl oz of gin
2 fl oz of peach schnapps
4 fl oz of grenadine
1 liter of lemonade
1 large orange
1 large lemon
a lot of ice