The life insurance cover only covers the period stated on the insurance document, so generally until retirement age. The chance of an individual of working age being diagnosed with a critical (though not necessarily terminal) illness is greater than the chance of that individual dying over the period of cover (i.e. generally before they are 60/65).
And, you can be diagnosed with multiple critical ilnesses during the period of cover. As long as the original policy allows you to continue with the policy if it pays out once, you would be entitled to claim again if you are diagnosed with something else, or you could claim multiply and simultaneously if you have a number of different injuries or illnesses that prohibited you from work under the different Sections of the cover.