Whenever you set up a company you need to set up a share structure. If you set it up publically and offer 100 million shares for sale, then simply put, people start buying them and when their all sold then thats it. There are no more. From then on in, the only shares you can buy are shares that someome bought earlier and they've decided to put up for sale typically for profit. If the company restructures its shares later and offers a private placement of , for example 3 million shares then it increases to 103 million shares, as well is there are warrants attached to those shares then the fully diluted moves up 3 mil.

Remember that the process is far more complicated than what I have illustrated but it s my layman way of explaining it.