You need to set the number argument for get sites, unfortunately, it does not seem to be a way to set it to all by using -1 like in WP_Query(), instead it seems that you need to set it to a number that is higher than the number of the sites you have.
The reason it’s returning 99 is that the max is 100 and we’re eliminating site 1 (the main site)
$sites = get_sites(array('number' => 500));
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