most (not all, some contributions are QTVR-only due to historical reasons)
many, but depends on photographer's choice of file upload (newer event = more likely to work) and your device's capabilities. You may experience slow playback, distorted images or unexpected browser quitting if there is only limited memory/GPU power available.
Since the birth of our daughter in December my wife has joined a mothers' group (people living in the same area with babies of the same age). They take it in turn to host the weekly group's meeting. What a great little community !