There is no hard limit documented. But you may hit the limitation of Power BI Desktop as Windows application first (out of memory), when you import a lot of data. Also, keep in mind there is a hard limit of 1 GB for .pbix file size that can be published to Power BI Service (the .pbix file itself can be much larger). I think they are working at Microsoft to make this limit higher in the future.
Just to give you an example - some of my clients work with tens of millions of rows imported into Power BI Desktop file. But if you hit the limit - if your desktop machine goes out of memory - you can move the data to one of the databases for which Power BI supports live connections (DirectQuery, more: https://powerbi.microsoft.com/en-us/documentation/powerbi-desktop-use-directquery/).
There is no official document points out the maximum number of rows imported to Power BI desktop. I have sent a email to consult this from PG internally, will keep you updated if I get information.
I got the information below internally:
Neither Power Query nor Power BI Desktop have a strict limitation on the number of rows.