The water you see on the surface of the planet is only a tiny fraction of the total water content of the planet. For comets to have brought all the water to the planet, many many millions of them would've had to impact the Earth over millions of years. I won't go into the details of the why's and wherefores of the competing theories and possible scenarios as to how the water got here and the formation of the inner planets, suffice to say that the answer hasn't really been found yet. There are problems with planetary bulk compositions and the range of temps that may have been found in the inner solar system at the time, as to whether the water was present "native" to the inner solar system or brought in from outside, apart from other factors as well. It was probably a combination of both, but that's yet to be determined with any certainty. However, if you look at the bulk composition of our planet (well, most of it's mass anyway), it sits smack dab in the middle of the siderophilic chondrites in mineral composition. There's also a significant amount of carbonaceous chondrite as well, which is where the water would have come from. The other form of chondrites (siderophilic) tend to be rather dry, usually less than 1% water. What's most important to the formation of the inner planets was the Oxygen to Carbon ratio, which for our solar system is about 6:1 in favour of Oxygen. The inner planets would've been a lot dryer if the ratio had've favoured Carbon. Any water that would've formed then would be locked up in hydrocarbons and the seas on Earth would've probably been petroleum instead of water!!!. As far as your second question goes, there were (and still are) trillions of comets in the solar system. More so back then though, especially in the Kuiper belt and inwards of Jupiter, in the main asteroid belt. Most of the cometary material now resides in the Oort Cloud surrounding the solar system. Much of the material that is presently there once resided in the solar system itself but the gravitational influences of the planets flung much of the debris out into the area where the cloud now is.
Hope that's helped answer some of your questions
