What would be the normal amount of alcohol to get if you were going to buy the alcohol for your reception of 300 people for beer and mixed drinks and champagne for the toast?

There are a lot of variables... the ages of your guests, the time of day of the reception and how long it will go, inside or outside reception, etc. Also, do you think more of your guests are beer drinkers or wine drinkers or mixed drink people. This will determine somewhat the variety of what you buy. Some people may also specify certain brands of liquor that they prefer and soda water, tonic water, etc. And many people will not drink alcohol at all, so you would also want pop, water, or punch, etc. for those guests.

Your best bet would be to ask the liquor store (where you will be purchasing the alcohol from) these questions. They know how many cups a keg of beer holds (you can get different sizes). You might be able to figure out a bottle of booze by dividing 1 1/2 to 2 oz. of liquor in the total oz. size of the bottle. 60 oz of Windsor = 30 to 40 drinks, a bottle of wine = 4 - 6 glasses, etc. You could figure 1-2 drinks per hour per drinking guest. Also check if you can return any unopened bottles or kegs for your money back.

For the champagne toast, you can probably get 7-8 glasses per bottle (depending again on the size of the bottle). So you need to figure out how many people will get champagne glasses (children probably won't). Also you may want some non-alcoholic for those who do not/cannot drink alcohol.

Or, if you are having the food catered, they may have good guestimates. You may consider hiring several bartenders for 300 guests, in which case they will probably know the mix that is appropriate.