If you are only talking about V and V7 chords remember the Letter is the scale degree the chord is built on and all chords are 3 notes. Any additional numbers next to the letter are added instructions to build on the basic 3 note chord.
EX: if you are in the key of F major. F is I, G is II, A is iii, Bb is IV, C is V, D is vi, E is vii. The fifth degree of the scale (V) is C. the V chord is C-E-G. If you want to make a V7 chord you add a fourth note C-E-G-Bb.
Sometimes you might also see "V/V7" which is secondary dominants. This is different than simple V and V7 chords. But just as easy. In this case you would take your original key, F; go to the V which is C, then go to C's V which is G and build a 7chord (G-B-D-F). Note even though Bb is in the key signature we make the V/V7 a major chord so must raise the Bb to B-natural to get our major chord.