Give each requested item an identity
An email saying “please send the remaining documents” is hard to reconcile automatically. I would create a separate record for each approved request: matter identifier, requested item, responsible person, request date, target date supplied by the firm, and current state. The wording comes from the legal team. The software does not invent a deadline or infer a requirement from an unrelated matter.
The state model can stay small: requested, received, needs clarification, accepted, or withdrawn. Received means something arrived. Accepted means an authorized reviewer confirmed that it satisfies the request. Keeping those states separate prevents an upload event from becoming a false completion signal.
A request can also have several supporting files. A bank statement might arrive as one PDF or several scans. I would link the files to the request without forcing the number of attachments to match the number of requested items. The reviewer sees the association and can correct it.