I used 4 certs on JFK-CDG. My dates were somewhat limited and my objective was to fly round-trip in Mint both ways. After about 20 purchases and refunds, I finally found a CDG-JFK for 2 certs and paired it with my preferred JFK-CDG for 6 total. Except for this one flight, every segment I purchased on this route (either JFK-CDG or CDG-JFK) was asking for 4 certs. I found some BOS-CDG and CDG-BOS for 3 each but couldn't find a round-trip that worked for me that totaled 6 or less. My wife was able to use her 4 certs on BOS-CDG-BOS though, so it is possible to find them for 2 segments.