Pick a bun
Choose a recipe and then scale the recipe up or down depending on the number of buns/rolls/loaves that you might require for your next sandwich.
Bun Calculator (beta)
Note: if you try to scale a recipe down and the yeast gram amount goes below 4 or 5 grams, your dough is going to take a really long time to rise.
If you really need to make just one bun/roll, I would suggest just adding 6 to 8 grams of yeast and ignore the calculated amount.