I advice you to look for great palettes, like DawnBringer's 16 or 32 color palette. You can also use pixel art pieces that depict similar themes/subjects as inspiration, but don't just copy a palette without permission.
There are no explicit rules regarding the amount of colors an artist can use. For a piece this size, 16 colors or less is a good starting point.
Check the Pixel Art Tutorial over at PJ forums: pixeljoint.com/forum/forum_posts.asp
Hi Taertage,
Thanks for the tips, do you have some pallet colors?
How many colors can I use?
Thanks 
Hello 0geo0!
That is quite a spooky scene. It appears the dude looks back at his gruesome deed with some regret: He points his lantern onward into the darkness, but stops too look at the ax. Some points to improve:
- You use single pixel for the blood, but you use 2x2 pixel for the rest. You should use only one pixel size (1x1).
- There 133 colors, many of which are redundant. You should never use automatic tools like the lighting effect of the lantern: It adds many colors and you lose pixel-level control.
- Many of the lines and clusters/shapes are irregular or jaggy. I recommend reading about lines and line-art in pixel art.
- As for the lighting, the ax should have more form shadow on it. The pool of blood catches a lot of light, whereas the floor on which it resides doesn't.
- The background and composition is less than optimal. You can really use the background and frame to add to story.
Keep on practising, and feel free to use the rescoures and help over at the Pixel Joint forum 
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Thanks a lot :D