Some competitions limit the number of pieces that can be entered by one needle artist, primarily because of space limitations. There are competitions that will require you to send a photo of the completed project, and they select the final entries from those photos. In most shows, there will be specific rules you must follow. Some may have to do with size, meaning the needlework cannot exceed certain measurements. Some competitions require no glass be used on any framed piece.
Most competitions require that framed pieces be ready to hang.
You will also have to be sure your stitchery meets the criteria specified in any category. In some cases, needlework that doesn't meet requirements may be refused. In others, the needlework may be shown but will not be considered for premiums or prizes. I can give you an example here. Soon after 16-count Aida became widely available, I stitched a Saturday Evening Post cover on it, framed it, and entered it in my local county fair.