How to make one MergeMap Observable sequence to start a new one after one is completed?

I need to upload a list of files while ensuring no more than 5 files are uploaded simultaneously.

While my code already limits concurrent uploads to 5, it waits for all 5 files in a batch to finish before starting the next set.

from(waitingFiles)
.pipe(
mergeMap((file: UploadQueue) => {
return this.doUpload(file, documentId, documentVersion, saveDocument)
}, batchQtnFiles),
bufferCount(batchQtnFiles),
tap(() => {
this.updateQtyPending(saveDocument, documentId);
})
).subscribe();


Instead, I want it to immediately start the next file as soon as one upload finishes and always maintain the concurrency limit of 5 (no more than 5 active uploads at any time)

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