Article: Ertuğrul, the father of Osman I (founder of the Ottoman Empire), arrived in Anatolia from Merv (Turkmenistan) with 400 horsemen to aid the Seljuks of Rum against the Byzantines. After the demise of the Turkish Seljuk Sultanate of Rum in the 14th century, Anatolia was divided into a patchwork of independent, mostly Turkish states, the so-called Ghazi emirates. One of the emirates was led by Osman I (1258–1326), from whom the name Ottoman is derived. Osman I extended the frontiers of Turkish settlement toward the edge of the Byzantine Empire. It is not well understood how the early Ottomans came to dominate their neighbours, as the history of medieval Anatolia is still little known.

Now answer this question: How many horsemen did Osman I's father bring to aid the Seljuks of Rum?
400 horsemen