Like everything else in quilting, the answer is "that depends". If the quilt is just to make you happy or for charity or utilitarian, then anything goes.
If you're planning on making it for a customer, entering it in a show, or are just particular about how you like your quilts, then you probably either want to built unity in there somehow. Since it's a sampler, you can't rely on the piecing to create unity. I'm not sure how you plan to quilt it, but that could potentially help add unity. Finally, you can add unity by the fabrics you pick. Here are different ways to do that:
1) Choose a LOT of different colors! Go with all solid fabrics (or near solids) that have roughly the same amount of saturation of color. If you pick all low-saturation fabrics (like pastels), you want to add some contrast in there with black and/or white, but your white should be off-white/ivory, and your black should be a slightly dusty, "off" black. Likewise, you could pick high-contrast fabrics (like brights or neons). In the past, those used to always be against a solid black background, but currently the trend is to add contrast by using grey, beige, yellow or tonal colors to add interest instead of focusing solely on unity.
2) You could pick a split complementary color scheme & get 4 different colors out of it, plus as many tones/tints/shades of each of those 4 colors as you'd like. I think you'd be surprised how many different fabrics you can include even with "limiting" yourself to 4 colors (& all their iterations).
3) You could pick a multi-color print for use in border or sashing or some block where you use a fair amount of it -- then look at the colors shown on the selvedge & pick coordinating fabrics that reflect any or all of those colors.
4) Find a picture of nature (flowers, mountains, underwater, anything) that you really love & pick any or all of the naturally occurring colors in the photo. If they appear together in nature, they will almost always work together in a quilt.
Yes, it's true that many times samplers I see are stitched only in 2 colors. The reason for that is typically for one of two reasons:
(1) samplers are typically instructional pieces. Quilters who are stitching a sampler for practice do not want to spend money on lots of beautiful, expensive fabrics for a practice piece.
(2) sometimes people use samplers to show their skill at piecing and/or quilting & generally precision piecing is more noticeable with high contrast solid fabrics. They could do a different pair of high contrast fabrics for each block, but that would again, put the focus more on the fabrics rather than the skill of the piecer. By keeping the fabric choices simple, the perfection of each block is what grabs the viewer's attention.
But if you want to make your sampler with 200 different fabrics, there is no one to stop you.
