Complex sentence: The navy has had a requirement for a minimum of 11 aircraft carriers, but dropped to 10 currently deployable and one, the, is currently undergoing extensive systems and technologies testing until around 2021.
Split into simpler sentences: The navy has had a requirement for a minimum of 11 aircraft carriers. Currently there are 10 that are deployable and one, the, is currently undergoing extensive systems and technologies testing until around 2021.