There are many different types of ocarina as well as sizes, colors, holes numbers, shapes, materials, everything can change. However, whichever you choose, the only thing that matters is how the ocarina sounds.
But measuring an ocarina’s sound quality is not easy, except if you are a professional. So beginners should, at least, pay attention to the number of notes that an ocarina can play. This is why you may notice that ocarina sellers often indicate the lower and higher notes.

Let look at an example:
STL 12 Hole Soprano Ocarina "Enigma"

Size: 4 inches

Pitch: A5 to F7 including sharps and flats
This ocarina can play every note between A5 and C7, so you can play: A5 - A5# - B5 - C6 - C6# - D6 - D6# - E6 - F6 - F6# - G6 - G6# - A6 - A6# - B6 - C7 - C7# - D7 - D7# - E7 - F7

So this ocarina covers more than an entire octave, it is a real musical instrument. But it is also limited; some ocarinas can play two, three even more successive octaves. See what song you want to play with an ocarina, search the associated music sheet, if every note can be played, this ocarina may suit you. If not, try another ocarina.