1 tablet is equivalent to 1 teaspoon.
Just be cautious with the "tablespoon" measure though, because North American tablespoons only have 3 teaspoons, unlike ours, which have 4 teaspoons. So, if it's a North American recipe, you'll need 24 tablets. (1.8 carbs instead of 12 if you were to use granular Splenda. )

Cheers.

Rosebud