Bake pound cake in half sheet pan?
What pan size is your original recipe for? Roughly speaking, for a loaf pan recipe, you will need somewhere between double and triple the recipe.
Also, keep in mind that the sheet pan cake might need a lower temperature and definitely a shorter baking time.
Agree with Angela's comments.
To figure this out, consider:
If your original pound cake fit and 8x4x2.5" pan, that capacity is 6 cups.
If a 9x5x3" loaf pan, that capacity is 8 cups.
Safest to measure your sheet pan volume with water, but I think one 12x18x2 may have 25-30 cup capacity.
So, make a multiple of 3-5x the original recipe.
PS There are cake pan size charts out there, usually a big help in converting recipes, but none I looked at had sheet pans.