That’s tough. - You would need one counter per user per “Request for Evidence”. The only way you could do that would be to have a bridging object with a single field being the counter and you would need many 2 one relationships to both the user and the Request.
I am sure there is another way to do it, but the only ‘standard’ one that comes to mind is whenever you create a user you create a bridging object with a relation to all existing Requests - and similarly whenever you create a Request you create a bridging object to all existing Users - and I suppose when you delete a user, you delete all related bridging objects (and similarly for Requests! More than a few rules!
The other way would be with a custom event processor that created the bridging objects and relationships either as above, or only when a user updated the field the first time!