expression
as soon as capacity is reached; once the maximum number of participants is reached
Fixed expression used in announcements or notices to indicate that something will close or stop accepting entries when the limit is met.
定員になり次第、受付を終了いたします。
We will close registration as soon as capacity is reached.
このセミナーは定員になり次第、締め切らせていただきます。
This seminar will be closed once the maximum number of participants is reached.
Compound of 定員 (capacity, fixed number) + に + なり (continuative form of なる 'to become') + 次第 (as soon as).