NVMe for R640 10 Bay SFF Server

Hello all,

If I want to utilize NVMe drives for my R640 10 Bay SFF server but my chassis isn't the 10 Bay NVMe chassis, how many NVMe drives is my server capable of using?  2 NVMe drives or 4?  Was hoping to use 2 for storage and maybe 2 more for OS..?

Thank you!