Common transmission types for the 1999 Tahoe include 4L60E (4-speed automatic) and possibly a few others depending on the engine and trim level. Each of these has a different capacity for ATF (Automatic Transmission Fluid).
Therefore, I cannot give you a single answer. You need to identify your Tahoe's transmission type to find the correct fluid capacity. Then, you can look it up in a repair manual for that specific year and model Tahoe, or search online using the transmission type and "fluid capacity."