If you're talking about the primary failures to tolerate (PFTT), then in a stretched cluster your choices are 0 or 1. Regardless, when you have a stretched cluster you still must have a witness as it's not optional.
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If you make your cluster a 4+4+1 you cannot RAID5 across the sites as that requires 4 Fault Domains - this isn't increased by adding more nodes, you still have SiteA + SiteB + Witness.
With more nodes (at least 4 per site), you can use RAID5 local protection on each site (e.g. PFTT=1,SFTT=1,FTM=RAID5) - this does still require a Witness though.
Also you could have just RAID5 pinned to one site (PFTT=0,SFTT=1,FTM=RAID5,Site=A/B) with only data on that site if you have data that doesn't require protection across sites - this doesn't require witness for data placement but the stretched cluster does of course still require a witness.