There is no rule for color count but the art itself guides it. Here you have a very lo-res mockup so loads of colors undermine the image to pixel admirers on a technical level. I would think this could be done with 24 to 32 well chosen colors with no visual difference in quality.

Anyhoo, you do have to change the preview file to 1X pixels, k.

there isnt a limit for anything, but in pixelart its considered good form using as less colors as possible,.... every pixel should be placed with highest care and so should the colors 

besides that i rly like the piece, looks very clean ...nice job.... but i think the seagulls look more like ducks,  maybe make the beak more orange and give them grey wings... like so

Oh, what should the colour count be? Is there a limit?

Why this is 117 colors makes no sense. Most likely you have used transparent layers or your file output is wrong. While the mockup looks like fun on a technical level the color count ruins the whole thing.

Also, the shrunken, truncated preview/small file is not allowed in the PJ gallery, you need to correct that.