License requirements depend on your licensing model.
With an ingest-based license you only need a license based of the amount of data you're ingesting. It doesn't matter what your architecture is - you can do everything on an all-in-one setup (as long as it has enough capacity) or you can have a multisite cluster with dozens of indexers and search heads - all within the same license.
With a resource-based licensing your environment is licensed by processing capacity so more indexers-bigger license you need.
As this is not your first architecture/licensing-related question I will again advise you to consult your business needs with your local Splunk Partner - there might be other issues worth considering before choosing the final architecture.
Unless you're working for your local Splunk Partner wannabe - in such case ask your employer to invest in Architecting Splunk Environments training for you. 😉
