Hi HDawood,
1] Yes, the EX4600 is shipped from the factory with two power supplies.
Each power supply is a hot-removable and hot-insertable field-replaceable unit (FRU) when the second power supply is installed and running.
2] If you are going to purchase another EX 4600 to setup Virtual Chassis, that too will be shipped with 2 Power Supplies. I believe you can purchase another power supply a spare, also note that if you have a valid support contract then you could get a replacement for the failed power supply.
For more details wrt EX 4600 Power Supply, refer : https://www.juniper.net/documentation/en_US/release-independent/junos/topics/topic-map/ex4600-power-system.html
Hope this helps. 
Please mark "Accept as solution" if this answers your query.
Kudos are appreciated too!
Regards,
Sharat Ainapur