[quote=Mtntopview]
MehMehOka okay, seriously:How many can cause a massive explosion capable of destroying an entire city?
[/quote]

Bob is 3S, which is how I measure destructive force. With a single attack at full power, Bob is capable of wiping out 3x the mass of a star. For this I use the size of our sun as a baseline, as the size of stars can vary immensely. One star has the mass of about 1.200.000 earth-sized planets. In total, Bob has a destructive force of 3.600.000 planets.
I would estimate Unicron to be at about 1S, although as he's absorbing the star, it doesn't really count as a single attack.
Galactus is about 10G, which is 10 galaxies. Based on a modern estimation of the amount of stars in a galaxy, 1G is about 50.000.000.000.000S or 60.000.000.000.000.000.000 planets. That should give you some notion of scale.