The complexity of the quantum state of a multiparticle system and the maximum possible accuracy of its quantum description are connected by a relation similar to the coordinate-momentum uncertainty relation. The coefficient in this relation is equal to the maximum number of qubits whose dynamics can be adequately described by quantum theory, and therefore it can be determined experimentally through Grover search algorithm. Such a restriction of the Copenhagen formalism is relevant for complex systems; it gives a natural description of unitary dynamics together with decoherence and measurement, but also implies the existence of a minimum non-zero amplitude size, as well as a restriction on the equality of bases in the state space. The quantization of the amplitude allows us to formally introduce a certain kind of determinism into quantum evolution, which is important for complex systems.