Not enough records to know for sure but many estimates put the death rate in the 2% range.
Not everyone had the flu but remember there was much less ability to care for those who did get it.

Tried looking for an authoritative site to check my understanding of mortality rate and found this which says 1/3 of world population got the 'Spanish Flu' and deaths were estimated at 50 million. Now if I did my math correctly from the numbers at this site only 1% of those who got it died.  I do know there are other estimates that are higher as I am sure I have seen it often quoted as 1-2%
https://www.cdc.gov/flu/pandemic-resources/1918-pandemic-h1n1.html
   
It is estimated that about 500 million people or one-third of the world’s population became infected with this virus. The number of deaths was estimated to be at least 50 million worldwide