Besides buying that much of BTC together will mostly break the whole market. it could be chaotic for entire crypto market if that much money inflaw into it.
So don't worry this rumor will not become reality. Maximum they can invest is $1 billion in Bitcoin which will be a very positive thing for market.
Yep I agree, the total of their wealth fund is 776 billions.
#The Public Investment Fund (PIF; Arabic: صندوق الاستثمارات العامة) is the sovereign wealth fund of Saudi Arabia. It is among the largest sovereign wealth funds in the world with total estimated assets of US$776 billion (£538 billion) #
https://en.wikipedia.org/wiki/Public_Investment_Fund