It’s not really a set number. It will depend on the in house equipment. The ONT really isn’t a factor. If your talking FiOS TV One then one DVR server can feed four IP clients. If you get premium DVR it can be two DVD servers and eight IP clients for a total of 10 HD boxes. You can also add to that by getting their digital adapters, but he are only SD.
Older Motorola/Arris 7000 models can do 15 HD boxes but you need to have an Actiontec Rev I router. You can also add digital adapters to the mix as well.