Can someone explaon how can I reconstruct the original entities back if there are, for example, more than one person names?

You cannot — not with the model alone. It gives you spans + types, not identity.

You need to do that part yourself after the model runs. The filter gives you spans; for each one, assign a stable ID (PERSON_1, PERSON_2) and keep {PERSON_1: "Harry", PERSON_2: "Ron"} next to the document. Swap IDs in before the LLM call, swap originals back in the reply.

Scoping that map to a document/project keeps the same person consistent across calls, so Harry stays PERSON_1 instead of becoming PERSON_3 the next time he's mentioned.

(Disclosure: I'm building a Mac privacy tool, RedMatiq, that does exactly this. The mapping layer turned out substantially harder than detection.)