When the rebellion began, the Kingdom of Granada counted barely 150,000 inhabitants, most of them Moriscos. The exact number who rebelled is unknown, but the ambassadors of France and of the Republic of Genoa at the Madrid count estimated that there were 4,000 rebels in January 1569 and 25,000 by the spring of 1570, of whom some 4,000 were Turks or Berbers from North Africa who had come to support the rebellion. On the other side, the royal army had at the beginning 2,000 foot-soldiers and 200 cavalry under the command of the Marqués de Mondéjar. The number increased substantially when Don John took charge: in the siege of Galera he had 12,000 men, while the Duke of Sessa at the same time commanded between 8,000 and 10,000 men. From its start in the Alpujarra, the rebellion spread to the plains and to other mountainous regions on the edges of the Kingdom. A particularly dramatic conflict took place on the ridge  above Frigiliana, in the Axarquia, where entire families of Moriscos from all around had gathered: the siege lasted from June 1569 till September, when Spanish reinforcements were brought in by sea. Moriscos living in the towns—including the capital, Almería, Málaga, Guadix, Baza and Motril—and their surrounding areas did not take part in the uprising, although they sympathised with it. This distinct attitude of the towns can be explained by the presence of a greater number of "Old Christians" and better integration of the Moriscos in these communities. On the other hand, in the Alpujarra and other regions, where the rebellion caught on, there were villages where the only "Old Christian" was the parish priest.
Question: How many more rebels are thought to have joined by the spring of 1570?
Answer: 21000