Corals repruduce in many ways, depending on species. For example asexually, like fragmentation or buddying, or sexually. Then there are many different ways of sexual reproduction. Some release egg- and sperm bundles, like Acropora. Others can release sperm and have internal fertilization and then release a larvae ready to settle, like Seriatopora. They can also self fertilize. These are just two examples, there are more ways too in other coral groups. Really cool stuff to read up on :)
Many corals probably spawn, release eggs and sperm, in our aquariums but not many get fertilized larvae or get to the stage that they settle. Most common stony coral to successfully reproduce and settle in aquariums is Pocillopora damicornis I think.
In some corals like Acropora, size is an important factor if they produce gamets or not. In the litterature it says they need to be above a certain size to put their energy into reproduction.
It doesn't have to be more than one colony to release gamets/spawn in a tank.