"Score" vs "number of individual" plot in GA algorithm meaning?

Hello,

I have a question about a plot that MATLAB creates using GA algorithm. it is "Score" in the horizontal axis and "number of individual" in the vertical axis. I would like to know what each of these means or reperesents? I saw in Mathwork that the number of individual reperesent the number of population in each generation. Based on my understanding shouldn't the number of population stays the same in all generations?

Also is the mentioned plot for only one iteration or the complete optimization process?

Thank you in advance for your answer and time!