How many strings are there on a standard grand piano and when correctly tuned, how many pounds of tension should these strings be under in total?

Depends on the brand of the piano. There are normally about 230 strings (the lower register uses two strings per key, and the lowest octave or so has only one string per key). Each key when pressed exerts tens of thousands of pounds of pressure on the strings, and as there are 88 keys on the standard grand (Bösendorfer Imperial Grands have 100 keys), I am unable to do the arithmatic, but it is a very high amount of tension. Most of the tension is easily absorbed by the castiron frame.

According to the following site, a medium-sized piano does indeed have 230 strings with a combined tension of 18 tons.
In a concert grand, the total is nearer 30 tons.
http://www.pianoworld.com/facts.htm

Incidentally, in the unlikely event that you ever need to find middle C in the dark without playing the notes, it's usually just above the keyhole.