qubits n initializes a qubit register of size n. By default, all qubits are initialized in the state.
The maximum size of the qubit register is determined by the back-end and your account limitations.
The total memory allocated for your algorithm is determined by the qubit register size. Make sure you never allocate a qubit register larger than the number of qubits used in your algorithm. This will minimize memory usage and optimize execution time of the algorithm.
Together with the qubit register, a binary register will be created having the same number of entries as the qubit register and used for storing measurement results.