spy satelites

I have noticed while on holiday how many low-polar orbit satelites there are in the sky. I suppose everyone has seen many of these in the sky, away from light polution. They appear to be stars that fly at incredible speed across the sky, faster than any aircraft. They are used to refresh a picture of the globe every so often, unlike geostationary orbit satelites.
... but when I was on holiday I saw up to 4 in the visible sky at any one time, this is pretty shocking stuff.

How many could there be up there and what can they do!?

They are probally not spy satelites. SS can usually be found above governemtn projects, as no one owns any part of space [some people are selling bits of the Moon that they don't own].

So places like AREA 51 are being watched by Russians, other big HQs for government units are also being watched [CIA, FBI, Whitehouse, NSA etc...].

Places are getting harder and harder to "spy" on because the project owners are becoming a lot more careful. Area 51 only do stuff when they need to and you don't usually see much outside of the hangars, because they keep most good stuff inside [under] the base!

With a program like Home Planet - freebie, built by John Walker of Autodesk fame, you can load up the current element for most satellites and see just what is zipping by. Some classified stuff is in there too - just won't tell you what it does or what's it for!! - Nifty!!