It is impossible to fetch all of Stack Overflow's users in a single API call, or even in a single day (using just the API and one IP address).
There are currently 1,995,355 users on Stack Overflow. Which means that you would need 19,954 API calls to get them all, but your maximum API quota is 10,000 calls per day.
The smart thing to do is to have your app prefilter the user criteria as much as possible and just fetch data on the few users that meet the filter.
But, if you really want all users, then get them from the Data Dump. Alternatively, you might be able to get the usernames from the Data Explorer (SEDE). For example, use this sample query to get this sample CSV file. I haven't tried it for all 2 million users (that would be a 1 or 2 Gig CSV file!), but it might work. ;-)
The SEDE data can be up to a month old, and the Data Dump up to several months old, but use them to get the bulk of the users. Then you can use much smaller API queries to get users added since the last Data Dump update.
See this question for an app that successfully melded the Data Dump and the API.