Complex sentence: Their lifelong friendship, and most of what is known of Van Gogh's thoughts and theories of art, is recorded in the hundreds of letters they exchanged between 1872 and 1890: more than 600 from Vincent to Theo and 40 from Theo to Vincent.
Split into simpler sentences: Their lifelong friendship, and most of what is known of Vincent's thoughts and theories of art, is recorded in the hundreds of letters exchanged between 1872 and 1890. There are more than 600 from Vincent to Theo, and 40 from Theo to Vincent.