Must I purchase a special type of bedsheet for a mattress that is between 8-12 inches thick?

Answer 1

You might want to check the package for the dimensions to make sure that it will fit your mattress. A lot mattresses these days are a lot thicker especially if you have pillow-tops. Most sheets come with elasticized edges to make for a good fit, but not all sheets are created equal. You don't want too tight of a fit or too loose.

Answer 2

yes, buy deep pocket sheets or your fitted sheet will keep popping off or won't fit altogether.