Count How Many Reports Occurred Last Month Without Updating Each Month

Hi fionabee 
The best practice for this kind of calculation is to use date table and time intelligence functions.
Please refer to the linked article:

https://radacad.com/power-bi-dax-previous-month-to-date-quarter-to-date-and-year-to-date-calculations

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pls try this

Last Month Total New =
CALCULATE(
    COUNTA('Reports'[Team]),
    'Reports'[Created On (bins)] IN { EOMONTH(TODAY(),-2)+1}