Something isn't clear to me according to Cosmos' documentation that states:
Deletion of expired items is a background task that consumes left-over Request Units, that is Request Units that haven't been consumed by user requests.
My question, for example: If I have 1000 documents on Cosmos NoSQL that will expire their TTL in the next second, will it consume 1 RU for the whole delete operation or 1000 RU?
TTL is not the same as the deletion of the documents themselves.
The TTL is immediate. The deletion of the documents stored can take longer (and btw, costs more than 1RU) and will consume whatever unused throughput is available per second taking as long as is needed to flush the documents from storage.