1- Do we need license for using this feature?
Yes - see the other post where I mentioned the licenses 🙂 https://forums.juniper.net/t5/Junos/JFlow-on-MX960/td-p/480755
2- As I checked datasheet of MPC10E-C-MRATE and MPC7E-10G, this feature is supported on LC, do we need to install Multi-service MPC as well?
No - this is supported inline on the MPCs (linecards)
3- What is the benefit of JFLOW vs. IPFIX. Can I use IPFIX instead of JFLOW?
JFLOW or IPFIX is just the format of the packets. You can use both types depending on your needs (I expect you cannot use both JFLOW and IPFIX at the same time).
4- So If I choose using IPFIX can I omit JFLOW licenses?
No.