Only one. You need to install Reporting Services using Enterprise Edition tools, and avoid using all kinds of nasty things for licensing, as well as for a failover cluster. If you try to do this using standard SQL Server media, you cannot install SSRS in a “shared database”, and the encryption information in the database created by instance # 1 will mercilessly explode into oblivion. This will cause SSRS instance 1 to not work while instance 2 is running (on the second server). Trying to fix this will only turn the situation around.
Personally, I think this is pretty cheating from Microsoft, as it actually means that you need to weigh the cost of Enterprise Edition against the Reporting Services HA permissions.
, Enterprise Edition, , .
Sam T