It's contained in s21(2) RPA 1985. Interestingly, there is no time limit for the council to co-opt it seems. If the numbers of candidates does not meet the minimum number for a quorum after a full election, the county council (or whoever your elections authority is) must act to ensure there is a quorum within 35 days but the 35 days is not a limit on the time within which the parish council can co-opt. It does seem possible then that if a full election occurred in 2023, the council could still have vacancies that it can move straight to co-option to fill.