There is no hard and fast limit. A lot depends on the resolution and framerate of the NDI feeds, whether it contains alpha and the network configuration.
A 1080 @60 NDI feed uses about 150Mb/s of data on the network. That means that a dedicated switched 1Gb network can hope to run 4-5 1080@60 steams. (networks can run up to about 70% load if properly configured – not higher).
4K or UHD uses 4x as much so one 4K stream per 1Gb network or up to 10 x 4K on a 10Gb network. (Though we have never seen this many work well without a professionally managed network).
There is a great white paper about this from Newtek here.
If you use the maximum theoretical amount of NDI feeds on your network, the Hippo will need to process this data and performance may vary depending on what other tasks the system is also trying to process.