So if I got something like Sigma 50-500mm F4-6.3 APO DG HSM Lens and a 2x tube then that would be a decent set up for long distance?
The 50-500mm f4-6.3 lens is seriously slow to focus. By adding a TC to this lens, no autofocus and an even slower lens requiring very good light to get good images. The TC's will significantly reduce the performance of your lens.
Adding a 1.4x TC (you lose 1 f-stop of light) would create a 70-700mm f5.6-8 lens
Adding a 2x TC (you would lose 2 f-stops of light) would create a 100-1000mm f8-16
F-stops are 1, 1,4, 2.8, 4, 5.6, 8, 11, 16, 22
The fractional stops .....f/4.5, f/5, f/5.6, f/6.3, f/7.1, f/8, etc.
They are called fractional stops because they are 1/3 and 1/2 of a f-stops) if I've understood it right