The box says 5. I have 7 on there now. No issues yet.
The lights are spaced approximately 2.5'' apart.
YES. Plug one end / socket other end for chaining together.
It says a maximum of 216 Watts and each string is 40.8 Watts.
I have no idea how much electricity one string uses, however my seasonal electric bill was not significantly higher than expected.
Yes these lights are string to string which makes the connections very easy to set up!