You can have one (or a few) PNG sprite sheet and use it for multiple different sprites. You'll need to create the sprite object and use
loadGraphic(graphic,true) to load the sprite sheet. "animated" must be true; if it's false the entire image will be one static frame. "graphic" is the embedded PNG of type
You'll have to put them all in one horizontal row and each frame will need to be the same width and height as all the others. Ideally frames should be square, but you can specify rectangular frames. Then you can use
frame = value where value is the index of the frame you want to use for static images.
You can also do
addAnimation("name", [1, 2, 3], 29) to create named animations that you can play back with
play("name"). That said, it is probably a good idea to still do separate sprite sheets for animated sprites.