Yes, JioPhone is a dual sim mobile phone. You can use one 4G sim card and one 2G sim card.
No. There is no WhatsApp support in JioPhone.
Yes, JioPhone can work as a wifi hotspot.
JioPhones comes with 4GB Internal Memory - so just like a normal Android smartphone, you can save as many contacts depending on the free space in your phone. In practial sense, you can store unlimited contacts.