Is it sufficient to repeat after the muezzin once, or is it obligatory to repeat after every call to prayer?

The Prophet, peace and blessings be upon him, commanded following the mu'adhin by repeating what he says, sending prayers upon him, and then asking Allah for al-Wasilah for him. He made the reward for this remembrance his intercession, peace and blessings be upon him, on the Day of Resurrection. This following of the mu'adhin is recommended, not obligatory.

The general principle is that a ruling is repeated with the repetition of its cause. Thus, hearing the adhan necessitates following the mu'adhin and reciting the remembrances associated with it. Whenever the cause of hearing is repeated, these remembrances are repeated, whether by the renewal of prayer times or by the multiplicity of mu'adhins at the same time. It appears that whoever suffices by responding to one mu'adhin will attain the promised reward, and it is recommended to repeat the remembrance with every mu'adhin whose adhan one hears.