Hi Dagmara,
If you just need "the number" of Actual Hour for a Project, I'd suggest you build an Hours-based Matrix report, grouped by Project vertically, and Endry Date (by Month) horizontally, and consider adding prompts to it for Project and Entry Date (range).
If you then decide to store that number in a Project level custom data field (eg Hours Last Month), since "you can't go up" in Workfront reporting, you can manually type it in, or consider using our UberCalc solution, which could do so automatically, among many other interesting things.
Regards,
Doug