Set the option showcmd (:h 'sc'), and you will never ever need to type anything to know how many lines are selected -- at first, as I forget that I've set this option, I didn't understand the point of your question. ^^'

Otherwise, if you want to obtain that number programmatically, it's simply:

:echo line("'>") - line("'<") + 1

From within a range-function, it can also be obtain by a:lastline-a:firstline+1. (:h function-range-example)

Set the option showcmd (:h 'sc'), and you will never ever need to type anything to know how many lines are selected -- at first, as I forget that I've set this option, I didn't understand the point of your question. ^^'

Otherwise, if you want to obtain that number programmatically, it's simply:

:echo line("'>") - line("'<") + 1

From within a range-function, it can also be obtained by a:lastline-a:firstline+1. (:h function-range-example)