Complex sentence: At least 5,000 MP18.1 were built and used during World War I, based upon observed serial number ranges of captured weapons; however, it is possible that up to 10,000 were built for the war.
Split into simpler sentences: At least 5,000 MP18.1 were built and used during World War I, based upon observed serial number ranges of captured weapons. However, it is possible that up to 10,000 were built for the war.