Whilst you set-up Outlook Anywhere from the Exchange console, it's actually using the RPC proxy feature, which is separate to Exchange. Exchange doesn't actually care where the RPC connection comes from, whether its local or over http, as they all look the same when they get to Exchange, the RPC proxy and IIS handle the incoming traffic and forward it to Exchange.
To get some statistics on how many RPC users you have your going to have to look at the IIS logs. There is a good article here on using Log parser to get these kind of statistics.
For example, getting the RPC statistics would look like this:
logparser "SELECT cs-username, Count(*) AS RPCProxyHits from c:\windows\system32
\logfiles\w3svc1\ex*.log WHERE cs-uri-stem LIKE '%rpcproxy.dll%' AND cs-username IS NOT
NULL GROUP BY cs-username ORDER BY RpcProxyHits Desc" -rtp:-1