1. Is it atx or e-atx?
2. Can I use xi formula with cpu pin-8 only?
We aren't able to help you with queries about our motherboards.
However for more information about Maximus XI , please see our website;
but there is not detail
This is an ATX motherboard. It is listed on the specifications page. https://www.asus.com/Motherboards/ROG-MAXIMUS-XI-FORMULA/specifications/
It is considered best practice to connect all of a motherboard's available power connectors. Failure to do so can lead stability issues if not enough power is available.
As @CH_ASUS stated, in the future please only use this forum for ZenFone related queries.