It looks like you’re using an older version of the .NET management SDK, and per the older docs, you can do the following to get the full list of users.
// Loop through pages, 100 users at a time
Uri nextPage = null;
PagedResults<User> users;
do
{
users = usersClient.GetList(pageSize: 100, nextPage: nextPage);
foreach (var user in users.Results)
{
// Perform user action
}
nextPage = users.NextPage;
}
while (!users.IsLastPage);