There's no limit, but it takes a lot of time.... and if you don't intend to use or try the packs then there is no need to install them on the Genos.
Once you have installed the packs that you want, you can ADD new packs by WiFi (no USB!). It worked on Tyros5, the memory will not be formatted, the pack gets installed in a matter of minutes. I guess it only works when you ADD packs, not when you delete or uncheck packs or items in YEM.
Stijn