The performer presents a metallic tray upon which are seven coins — all quarters or half-dollars — all the same and large enough to be easily seen, anyway. S/he asks a member of the audience to assist; pours the coins into his/her hands and asks him/her to count them. There are fourteen! The procedure can be repeated twice more; there will be twenty-eight coins in all!
The tray actually has a double bottom, and is divided into four compartments, which open on to the four sides of the tray. In each compartment are seven coins, which are simply poured into the helper's hands by tilting the tray — which the performer will have to do anyway to get the visible coins over the rim of the tray.
A little misdirection is needed with this trick: to make it less obvious what's happening, the performer must appear to be tipping the same edge of the tray into the helper's hands. Thus a square, featureless tray is best. The performer advances the tray a quarter turn for each iteration of the illusion.
The double bottom must not be obvious, of course, and the helper must not be allowed to inspect the tray too closely.
The number of coins — seven — can be explained by numerological mumbo-jumbo. The number of iterations is four because the performer will protest that the money is coming out of his/her pocket and that's all s/he can afford! Or some other story will do.
Very adept manipulators will be able to palm a few coins at a time and add them to the pile as they are slid off the tray. This is preferable in one sense in that the number of iterations is not limited to four.