Hard to give a definitive answer here, so I will give you a few points that hopefully give you enough of an impression.

So Vietnamese communists were able to aid and train the original Khmer Rouge forces in 1970, who numbered around 3000, to more than 40,000 in 1973. By the end of the civil war there were probably more than 60,000 Khmer Rouge troops. Those numbers can probably be inflated to encapsulate the number of cadre who worked within a more administrative/infrastructural capacity.

After 1973 the Vietnamese presence in the Khmer Rouge was diminished significantly. But the Cambodian communists would begin 'standing on their own feet' and find recruits in villages that were damaged by US bombing, or those who supported the former leader of Cambodia - Prince Sihanouk. Few, if any recruits to the cause of the Khmer Rouge before 1975 were 'following Pol Pot', the leadership of the movement was still relatively secret to most people. Cambodians joining the Khmer Rouge during the civil war were fighting for a kind of social justice along the lines of communist propaganda/ideology.

'Choice' is a hard one to nail down here as well, as you may be aware there was little movement away from membership in the party or away from duties once the revolution was underway, actions which would no doubt indicate one's 'regressive characteristics' and lack of revolutionary zeal - inferring suspicion and probable cause for arrest/execution. So saying one freely chose to support the revolution with reference to some of the numbers i referred to at the beginning of this response may be a little hard to quantify.