As the cone cells are different in numbers in people, how can we say that everyone is seeing the color as same? for example the color you are saying as red may be not the one i see as red..
It is not known. Some would even go as far as to say it is not knowable.
"Color" is a quale; it is one of those things that may be different from one observer to the other, but both observers will always agree on the name, classification, etc. of a particular combination of frequencies and intensities of the EM spectrum. There is no way to discriminate between the two possibly different interpretations.
Physicists do away with this by saying something along the lines of "if there is no way to reliably discriminate between the two, there is no effective difference and it is pointless to consider it."
Philosophers, psychologists, etc. tend to worry about this a lot more. Personally, I think that some time in the not-so-far future it is possible to fully understand how one brain interprets for example "red", if and how it differs from how another brain interprets it, and eventually transfer the precise interpretation of the first brain into the second one, thus doing away with this ridiculous question once and for all!
The gauge to measure color perception by our eyes is the rainbow. The colors of the rainbow are only frequency dependent, thus the same for everyone.
Color blind people will confuse the blue with orange, for example, and this will be evident in what they label the rainbow colors.
I'd be happier answering this on the Philosophy or psychology stack exchange as I think there ARE actually at least DEFINITE answers to this, one that has a physics component, but an even more fundamental one that is not about physics at all, unless you count the signal processing that begets our consciousness as physics.
The physics component is given well by Anna V's answer and by Rody Oldenhuis's answer: that is that we can use a "standard" like the rainbow and show roughly that different observers will agree on the same colour, unless there is colour blindness. Actually it is more subtle than this, because the wavelength responses of two people's corresponding cones (red response in person 1 compared with red response in person 2, green in person 1 ....) can and is different in many cases. Indeed the green receptor responses split into two very distinct "populations" depending on how the relevant gene encodes the response (I've been looking around for a reference but cannot find it so far). Therefore, when two different spectra can be told apart by one particular person, another can't tell them apart and contrariwise. Some people see traffic light green LEDs as bluish green, others as greenish blue, and the difference is set by which of the two green receptor gene populations the observer belongs to.
Now for Rody's statement:
... it is one of those things that may be different from one observer to the other, but both observers will always agree on the name, classification, etc ....
I believe, psychologically speaking, we can go further than this. Two observers agreeing on the classification certainly will in general have a different experience of a colour, and we can prove this in a simple psychological experiment. How? Ask different people what their favourite colour is! People have different favourite colours: the conscious experience of the same pink will by someone who loves this colour is manifestly different for another observer who loathes the right same spectral mixture of light, even though both agree on the classification.