Limits are going to be incredibly tight on the short film script, and this includes making the limits applied to feature screenwriting tighter as well as applying new ones. There should only be one main character in a short film script and this should be relatively clear. In a feature screenplay you have more time to develop different characters, play them back and forth, and allow them to emerge as the dominant one. In a short film script people really only have a few minutes to be able to identify who they are following, why, and what the problem is.
Likewise, the problem should be pretty well identifiable right from the start. A short film will require you to establish what the issue is, which is where the characters are taken out of their normal pattern of behavior, and then lead you to resolve it. This does not have to be insultingly obvious, but it has to have a practical nature because you do not have the time to establish more esoteric motivations.
The number of characters overall should end up being limited in a short film script as a whole. This can mean that you only have a few characters and then under ten speaking roles, though there is not an iron clad restriction that would be set to this. You should also have a relatively clear correlation between scenes, where one scene leads into another. These scenes should establish that something happened and that there is a need to move into the following scene. This is always important, but you do not need direct correlations in feature screenplays since you can go back to it later on.