It depends on exactly what you're asking.
The spontaneous radioactive decay of a nucleus involves only one atom. For example if you take an isolated uranium-235 atom and simply wait it will eventually decay without any interaction with anything else.
However if you're trying to make an atomic bomb this can't be done by relying on spontaneous decay. Instead we use stimulated fission, where a uranium-235 nucleus is struck by a neutron from an external source and this triggers fission. The fission releases three neutrons and these strike other uranium nuclei and trigger their fission, and so on causing a chain reaction and nuclear explosion.