from your sample data, you just can DISTINCTCOUNT() the teamname for each employee. This would provide the number of changes. Due to the fact that the column Month will be considered as categorical variable with no implicit sequence you can't calculate how many changes have happened until the current Month.
You may consider to change the Month column to the datatype date, eg. 2017-08-01 representing august 2017, now it would be possible to also calculate the number of changes until a specific date, due to the fact that dates have an implicit ordering.
Hope this helps somehow
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