If all 100 people are drinking martinis, you’ll need two to three bottles of vodka, and equally two to three bottles of sour apple. The more people whom you anticipate will imbibe, the greater amount is what you should buy. Not sure about the recipe for your drink, but I would presume you should pick up about the same amount of fruit juice for the mixers, possibly a bit more than the others.

Two or three bottles?? Nice try.

For the sake of argument, let’s assume that each drink has 2oz of vodka, 1oz apple pucker and 1oz pear juice. Each liter bottle of booze has about 33oz but it’s best to round that to 30oz to account for spillage, so each liter of vodka will make 15 drinks. So you need 7 (6.66…) liter bottles of vodka and 4 (3.33…) liter bottles of pucker and juice.
Once shaken with ice, this should produce a 4–5oz drink.

Cocktail, not Martini.

Average two drinks per person, 1 1/2 oz per drink, so 3 oz per person.

Is everyone going to be drinking the signature drink? I know if I was offered one of those, I’d decline.

Your caterer can usually help you with this kind of number crunching.

But a good tip is to buy your liquor from Bevmo. They’ll let you return any unopened bottles. Just be sure to instruct your bartenders to only chill and open one (or two) bottles at a time. (They won’t accept bottles with damaged labels.)

Congratulations on your son getting married!