JioPhone comes with 4GB Internal Memory. So, you can save as many as you want - depending upon the free memory on the phone.
No. It is mandatory that you have to use Jio sim card in JioPhone.
Yes, you can use Airtel sim card in the JioPhone but the Airtel sim card will work only on 2G network.
Yes, JioPhone does support Live TV. You just need to select the channel, which you want to watch and hit the button "play".