時々 and たまに: occasional actions and rare occasions
Read occasional shop visits and infrequent visits by a friend without assigning a fixed count or percentage to either expression.
Something that happens from time to time
時々, read ときどき, means sometimes or from time to time. 時々、この店で昼ご飯を食べます。 says that the person sometimes eats lunch at this shop.
この時計は時々止まります。 says that this clock sometimes stops. It specifies neither the dates nor a regular gap between stoppages.
An infrequent occasion
たまに means occasionally or once in a while. 遠くの友達がたまに遊びに来ます。 says that a friend far away occasionally comes to visit.
たまに外で朝ご飯を食べます。 says that the person occasionally eats breakfast out. In a comparison of the same habit, たまに can suggest a low frequency.
No fixed numerical threshold
The actual frequency suggested by 時々 or たまに depends on the situation and speaker. Neither has a fixed percentage or number of times per month.
Ask たまにというのは、どのくらいですか。 to clarify the frequency. A reply such as 月に一回くらいです。 then supplies an approximate count for that context.
Does たまに alone specify exactly once a month?
Show the answer and why
No. It identifies an infrequent occasion without a fixed count. Separate wording such as 月に一回 is needed for a concrete frequency.