I've read that 'used to' isn't used while we're saying how many times something happened in past.
So, we can say:
I visited him several times.
But we cannot say:
I used to visit him several times.
My question is, can we say this:
I used to visit him several times in a day/ month.
Actually, here the idea is of frequency in a single time i.e. month, day. Since the words 'day' and 'month' refer to an event, not the number of actions.