Honestly depends on what situation you're talking about =P However I will never not run my skirmisher skinks in groups of 2 x 5. (All of the other things depend on points level and game size, as well as the unit's role)
Welcome to the forum! As stated above, there is no hard rule set for unit size. look through the army lists section to see what people are running. If you have any questions specific to a list, let us know.
Yes, there are 10 per unit, two ranks of 5. I'm sorry if I caused confusion, I didn't mean two UNITS of five.