I looked for this information once and found out that there are no restrictions for geocoding, however, it is reasonable to rethink how you send requests to make them as small as possible.
Apple says:
Send no more than one geocoding request for any user action.
If you want to automatically update the current location of users (for example, when a user is moving), issue new geocoding requests only when the user has moved a considerable distance and after a reasonable amount of time has passed. For example, in a typical situation, you should not send more than one geocoding request per minute.
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