First I would break it down by those that pay for nodes and those that don't. The ones that don't pay for nodes but still have a lot of nodes are pretty strong. That's BTC, LTC, and DOGE. The PoS coins are hard to assess, other than Dash. Dash has 3259 paid nodes. Even subtracting that, it still has around 200 volunteer nodes, which is decent.
NMC is a bit surprising to me. Would not really expect that to have 200 nodes.
Seems to (incorrectly) show 0 for many coins that are not BTC forks.
I think "Volunteer nodes" are part of the problem not the solution.
Bitcoin incentivizes miners, but not full nodes - the result is a declining and centralizing full node network that can't scale as usage grows.
You need to incentivize all the users or you end up with infrastructure not strong enough to support large transaction size / frequency / storage requirements, for example Bitcoin's block size debate, and node incentivization problems e.g.
http://www.coindesk.com/adopt-node-project-aims-bolster-bitcoin-network-security/.
Dash solved the problem with the protocol paying full nodes a % of the block reward same as miners, resulting in a very strong infrastructure, ~3,000 nodes running on servers, compared to ~5,000 in Bitcoin, at < 1% of it's market cap.
https://www.youtube.com/watch?v=FY1mciGGhO4