The memory requirements are getting huge, 4600MB now on my (24h) node, so it's not easy to have a node up all day. Most VPS don't have more than 2G/4G and most new desktop computers are still sold with 4G, so that's why we see this node scarcity.

Count 1 node for Moo's pool, 1 for Minergate, 1 for the exchange (bittrex?), Smooth's bootstraping server, yours, mine. And some that may be ocasionally up.
(Obviously those are approximated numbers )