Because teak sealers are applied directly to bare wood, the first coat will naturally yield less than subsequent coats. Once the first coat saturates the wood, the second coat can only penetrate so much. Most teak sealer products will yield only around 200 square feet per gallon on the first coat, but will yield 300-400 square feet per gallon on the second coat (minus any waste incurred from brushes, rollers, spillage, etc.). Most teak sealers need 2-3 coats for proper protection, and should be reapplied when the wood shows signs that the teak sealer has worn away.
Unless specifically directed to do so, do not thin teak sealer products in an attempt to get more coverage.