RE: Segment build - received email at least 'number of time'
Hi Graham,
Behavioral segments query the interaction records stored in the marketing insights database.
The following image shows an example:

1.Create a segment and then select the Add Behavior block option:
2.Interaction name: Use this drop-down list to select the type of interaction the behavioral segment will look for.
--Email delivered: A marketing email message is considered to have been delivered if it was sent (not blocked) by Dynamics 365 Marketing and no bounce was reported.
3.Click Interaction filters.
--Select this button to show or hide filters for setting limits on the total number of interactions and/or a moving-window time period.
4.Set filters: Use these to set a minimum or exact number of interactions that a contact must have performed to be included in this segment.
-- In this example, the filter will find contacts who receive messages more than 4 times.
5.Additional restrictions:
--they enable you to filter results based on values for the selected type of interaction records.
If you don't have other settings, you can delete it directly.

Refer to the following documentation for more details:
https://learn.microsoft.com/en-us/dynamics365/marketing/segments-interaction