Watch the Counting Beats online guitar lesson by Rich Maloof from Acoustic Guitar For Beginners
Here we're seeing what two measures looks like. This will give you an idea of what you'll encounter on a standard piece of sheet music.
Each measure has four beats in it, and you'll see each beat highlighted at first. Then we play a full measure of Fmaj7 followed by a full measure of Em. As seen before, it doesn't matter if you're playing one "whole note," four quarter notes, eight 8th notes - each of these takes up a full bar.
Whether you're playing with other people or just strumming while you look at a piece of sheet music, you'll want to maintain a steady beat and keep track of where you are in the song. Once you understand beats and measures, you'll never get lost.