My parents are having a party to celebrate their anniversary and want champagne for the toast, but are unsure how much they need to buy. How many glasses will they get from a bottle?

When deciding how much champagne per person, allow four generous champagne glasses per standard bottle.
For a toast or dessert, allow one glass per person.
For cocktails or as an aperitif, you may need two.

One-half bottle per person should be sufficient for an entire evening.

see here

For 100 guests 1-2 cases of champagne should be plenty.

Thank you for your help clarebear.