It’s no myth, it’s just the application of Islamic history to the modern world; a rule of thumb. The origin, I believe, comes from Raymond Ibrahim, an Arabic language specialist with an Egyptian heritage giving him insight into Western and Middle Eastern world views.
His Rule of Numbers article can be found here: http://raymondibrahim.com/2013/05/28/islams-rule-of-numbers-and-the-london-beheading/. It relates to the expulsion of Muhammad and his followers from Mecca to Medina in 622, sacralised as ‘hijrah’, or migration. Its representation today is of Muslims migrating to infidel lands following the Koranic exhortation, “Whoso migrateth for the cause of Allah will find much refuge and abundance in the earth, and whoso forsaketh his home, a fugitive unto Allah and His messenger, and death overtaketh him, his reward is then incumbent on Allah. Allah is ever Forgiving, Merciful.” (Koran 4-100, Pickthall tr.)
In what became known as the ‘Meccan period’ he acted peacefully and cooperatively with the locals, building up converts to the religion he claimed to have had revealed to him, but garnering the enemies who expelled him.
Once in Medina his attitude changed, as the Koran’s violent Medinan verses show. “And slay them wherever ye find them, and drive them out of the places whence they drove you out, for persecution is worse than slaughter.” (2-191 ibid). He built up followers, forces and arms, then went on to conquer Mecca. The verses from this time have the consequences today of observant Muslims who are obliged to follow Muhammad’s model of behaviour and ‘striking terror into the hearts of unbelievers.’ The role of migration, peaceful behaviour and proselytising, then followed by violent conquest, have their echoes in today’s world. In Arabic, these three processes are known as hijrah, dawah, and jihad. Ibrahim’s hypothesis is to show how Islam conquers territory where simple armed conflict is not a viable strategy.
The ‘Rule of Numbers’ has applications other than that Ibrahim adumbrates. Democratic success comes from the appeal to a greater numerical cohort. The 1960s and 70s Muslim immigrants were not political or particularly religious, but formed a constituency that voted mainly for the Labour Party. Muslims worked up the ranks and obtained positions in civic management, councillors, mayor and MPs. Then, when oil-funded Saudis spread their conservative form of Islam in the late 70s, they had an established electorate clique. In succeeding years, migration has boomed, inflating Muslim proportions in every country worldwide. Proselytising has had a remarkable effect, particularly in its negative form, that of convincing host-country natives, the media especially, that Muslims are peaceful not violent despite evidence to the contrary; that critics of Islam misunderstand it, are ignorant right-wing racist bigots despite evidence to the contrary; and that free speech, women’s rights, Western history, morality and culture are negotiable conditions in the Left-wing socio-political environment that imbues Western society today. In its positive form, it purports to show that Islam’s rigid 7th century morality has a place in the modern world where there’s no support for opposition; that violence can be condemned even as Islamic influence grows; and that Islam is a viable mode of governance for the West’s future despite evidence to the contrary.
From this success, it can be seen that violence is not a necessary condition of conquest and Muslims can condemn it, without needing to point out that victory can be achieved by other means. All they need is numbers.