Always buy 2 skeins for a pair of socks when it says 50g ball, to be on the safe side buy 3 or 4 depending on the ankle length.
I'm a bit peeved about having bought 4 skeins of (50g each) of dk weight yarn to make ankle socks for the summer. I'd bought 2 skeins of fingering weight and made a pair of socks with some left over, so I thought with these I'd get 2 pairs out of each skein for a total of 8 socks as they were only ankle socks - NO WAY, JOSE! I had one sock only for each skein with a tiny bit left over. Must be the different weight yarn. I hate that. Live and learn. On the upside - at least I get to buy more yarn!