There's lots of options online for planning how many drinks you need for a party. On the heels of our write-ups of how to calculate that number by hand and an automated option from Dan Murphy's, readers pointed to another useful site, this time from liquor retailer 1st Choice.
The 1st Choice option is similar to the Dan Murphy's site, but as commenter Con pointed out, it also lets you incorporate kids, non-drinkers and designated drivers into the mix. None of these approaches is perfect, but they're a good starting point if you're throwing an event outside of your usual comfort zone. Thanks Con!