The 100 extras available to members are refilled on 1st January each year for anyone with a current annual or monthly paid membership.
This is a legacy situation. Extras were introduced early in 2015, just after the site was redesigned. At the time paid memberships hadn't been reintroduced after the major site changes.
When memberships were reintroduced under the new community ownership (on 1st November 2016), everyone that bought a membership continued to enjoy the benefit of extras. They were all increased to 100 extras on 1st January 2017. Any paying member will have a new set of extras on 1st January each year.