Set ASP.NET label's text dynamically using JavaScript

I want to change the asp.net label control's text based on what the user inputs into the txtNumOfPeople dynamically before submitting the form.  Obviously the code below is just a hack of what I want to do and nowhere near how it's done which is why I came here.  I'm not a guru at javascript nor with javascript and talking to asp.net controls so I'm trying to learn.  How would one call this and do something like this below?

function ChangeNumberOfPeopleSelectedLabel()
{
    var label = document.getElementsByTagName('lblNumberPeopleSelected');
    var txtNumOfPeople = document.getElementsByTagName('txtNumberOfPeople');
   
    label.text = 'You specified txtNumOfPeople.text people'
}