The answer to that: Don't pick them based on the language they are proficient with. If you find an incredibly sage person who writes webapps in COBOL, take them. The language is not going to dictate the success of your venture.
As for numbers, there are generally more PHP developers than Ruby developers, but you want a good developer, not just a developer.
According to what I have heardÖ finding a good PHP developer is much more easier than finding a ROR developer coz ruby is new and less developers available and they demand higher salary.
So suppose I have an Idea and I'm searching to find out my co-founder who is a developer. shall I go for php developer or ruby? what are the elements which i should consider? and how this choice will effect me as the CEO in the future? for example is it hard to find a ruby developer or PHP? etc....