The Crimean Tatar's invasions of Russia  began in 1507, after the death of Moscow's grand duke Ivan III, with the Crimean Khanate attacking the Russian towns of Belev and Kozelsk. Over the course of the 16th century, the outer border of the Wild Steppes was near the city of Ryazan, outside the Oka River. The main path for the invading forces to Moscow was the Muravsky Trail, running from the Crimean Isthmus of Perekop, between the basins of the Dnieper and Severskiy Donets rivers, and finally up to Tula. The Tatars would turn back only after extensive looting and kidnapping, the Tartars usually managed to penetrate 100-200 kilometers into Russian territory. Captives were subsequently sent to the Crimean city of Caffa to be sold into Slavery. As a result, the Russian population in the border regions suffered heavily. Each spring, Russia mobilized up to 65,000 soldiers for border service. The defensive lines consisted of a circuit of fortresses and cities. To protect from invasions by the Nogai Horde in the region between the Volga and Irtysh rivers, the Volga cities of Samara , Tsaritsyn , and Saratov  were founded. The most damaging invasions occurred in 1517, 1521 , 1537 , 1552, 1555, 1570-72 , 1589, 1593, 1640, 1666-67 , 1671, and 1688.
Answer this question: How many Volga cities were founded to protect from invasions by the Nogai Horde?
3