Complex sentence: The first birch bark letter was found on July 26,1951 by Nina Fedorovna Akulova, and at least 1025 have been unearthed thereafter--923 in Novgorod alone.
Split into simpler sentences: The first birch bark letter was found on July 26,1951 by Nina Fedorovna Akulova. At least 1025 have been unearthed since, 923 in Novgorod alone.