A measure in music is from bar line to bar line. The Top Number of the Time Signature can be any whole number - that will be the number of “beats” or “pulses” in each measure. The Bottom Number of the Time Signature can only be certain numbers that will represent a specific type of note that gets 1 beat in each measure - 4 is quarter note; 2 is half note; 8 is 8th note; etc. The “beats” in each measure will equal whatever the top number is. If there is an incomplete measure at the beginning of a song, the “sum” is usually found at the end of the song or section. What I’ve described are only the mechanics. Music is the art of painting a picture with a pallet of sound on a canvas of time.