NOTE: This information is good as of 7/22/2015 and is subject to change.

*** Update ***

I had posted earlier that this was specific to Free users, but it has nothing to do with Free or Pro. Reposted to reflect the accurate behavior.

*** UPDATE – 9/2/2015 ***

The limit was increased to 200.  See the following blog for more details.

Power BI Weekly Service Updatehttp://blogs.msdn.com/b/powerbi/archive/2015/09/02/power-bi-weekly-service-update-9215.aspx

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I was looking at the Power BI Community Site and someone had indicated that they cannot save a report.

Unable to save report

When looking into that Activity ID, it appears that we have a limit of 10 reports.  This has nothing to do with if you are a Free or Pro user, but rather a resource limitation based on your Dataset/Model. 

When you try to save the 11th report against the same Dataset, you get the error above.  If you create an 11th report against a different Model it will work.

Adam W. Saxton | Microsoft Business Intelligence Support - Escalation Services@GuyInACube | YouTube | Facebook.com\guyinacube