Actually, you can play more than that, but C and G (bugle key) are the first two. You can also hit "pedal" tones on a brass instruments, but this is harder on higher instruments, and doesn't sound as good. above the G is another C, followed by an E, G, A (I think?) C, and so on. This is referred to as the "harmonic series" and can be derived from shortening pieces of string in interesting ways.
Also, bugles are, iirc, generally keyed in G rather than Bb or C. In DCI (drum corps international) bugles are considered G-pitched brass instruments, but over the years instruments in other keys such as the trumpet (Bb).
Originally, bugles for DCI weren't allowed to have valves, but some were made with one valve that could basically change the key. These were permitted, but the musicians had to pin the valve either "on" or "off" so that they wouldn't be playing a valved instrument. Some people then made instruments with valves on the side triggered by the thumb so that they could move the valves without being caught.
The largest bugles are contrabass bugles, and are considered the marching version of tubas. They are generally just known as "contras". The standard marching tuba used to be the sousaphone, but it has fallen out of favour to the contra, which is (apparently) more comfortable. Here is a picture of one: