Muejezaat, Burhaan Aur Sahaba
First refer to following thread to understand the reason of miracle given to Prophets Discussion 16063 The conversion process in the initial years of Nabuwat was relatively slow because it the message was given in private. After persmission was given to openly invite people to Islam, there were relatively large number of conversion. the number of muslims were actually greater than 13. The miracle of Prophet Mohammad saw was Quran and even Kuffar e Makkah was impressed with the eloquence of Quran.