Essentially they create them as they go.

Total shares outstanding plus outstanding stock awards:

    Dec 2011 : 468 million
    Dec 2012 : 470 million
    Dec 2013 : 476 million
    Dec 2014 : 483 million
    Dec 2015 : 490 million
    Dec 2016 : 497 million
    Dec 2017 : 504 million
    Dec 2018 : 507 million
    Dec 2019 : 512 million
    Dec 2020 : 518 million

Existing shareholders lose 1% per year.

I'm not sure about Amazon in particular, but generally companies (especially tech companies) hold some percentage of stock in reserve for employee compensation. However, the company can also buy back stock on the open market either to take it out of circulation (and thus increase the value of outstanding shares) or use it for employee compensation. Finally, if the board (as a proxy for individual owners) permits it, a company can issue new shares for any purpose including selling to raise cash for operations, or giving to employees as compensation. This isn't free money, however. New shares tend to dilute the value of existing shares. So owners often prefer to raise money other ways, like debt that doesn't convert to an ownership claim in the way stock does.

I don't know Amazon's specifics, but I believe that's how it's typically done.

It doesn't have to be that large a pool as percentage of the company.