No, that depends on many other factors. The "V" in V6 or V8 refers to the way that the engine is set up. When you look at it from the front, it looks kinda like a V. The 6 or 8 refers to the number of cylinders. Generally, the more cylinders, the more power, but that also depends on the size of the cylinders. A V6 can be a bigger engine than a V8, despite having 2 less cylinders.
It depends on several factors. I'd suggest reading this
article, but if you're too lazy to read all of it, just skip to this