How many people own crypto currencies? 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.