I brew small batches all the time. For dry yeast, I figure out how much is needed based on the batch size, e.g., roughly 1/3 pack for 1.5 gallons, 1/2 pack for 2.5 gallons, etc. Just estimate. If you use a little too much or slightly not enough, it doesn't matter. Tape the pack shut and store the rest in the fridge for next time. Dry yeast keeps for several years. With liquid yeast, I make a smaller size starter and save a little of the starter in a leftover White Labs tube for future potential use. But dry yeast is certainly easier to quantify and use, and keeps for ages.