Is there an estimate on how many people in total own some cryptocurrencies?
How is this distributed among bitcoin and altcoins?
As Luca mentioned it is really difficult to measure, but there is a way to map Bitcoin addresses to IP address under certain conditions. Refer this paper
Title : An Analysis of Anonymity in Bitcoin using P2P Network Traffic.
Authors : Philip Koshy, Diana Koshy, and Patrick McDaniel.
Note: It analyses relay pattern of transactions. It cannot analyse transactions that are already confirmed in blocks.
Some companies publicly disclose their data and it might be possible to come up with a ballpark estimate. Blockchain.info shares the number of wallets used in their My Wallet service.
It is next to impossible to estimate.
Consider this scenario: You live in a deserted place with just phones/post offices. Using just pen & paper/calculator you derive a fresh new bitcoin address and its private key. You call any of your friends, who could be anywhere in the world, who have internet and you give them this address. They make a transaction sending satoshis worth buying milk and relay it to the network.
You could then go give away your privatekey to a milkman for buying milk. If he doubts you he can call any of his friends in the world who have internet and can confirm that that address holds balance.
You and the milkman are untraceable unless your friends decide to give you up. No technology can do it. Well, maybe some neuro-tech can scan their brains but ..
I could travel across borders holding my money in my mind.
As of Today, BlockChain's MyWallet has about 3.75M users. Adding this with CoinBase's user base of 2M and Multibit's 1.5M. That makes about 7.25M Bitcoin users. Note that actual number of users may be much above this figure as you can figure out all that is needed to encrypt the data is just a key!