Hello Haochengdong Haochengdong,
A 2-rank system is implemented on the EVK and recommended as it may provide better performance and it allows to use the T topology to mirror the chips and make it easier to design, although a single rank design is also possible, as Mrudang mentions.
However, in general terms I would recommend that you follow the recommendations for DDR design on the i.MX8QM Hardware Design Guide (link below, please note that you may need to login to download this document) unless you are very confident in DDR4 design as routing DDR4 memories can be challenging.
I hope this helps!
It is possible to design 8MQ DDR4 PCB routing topology for a 1-rank system as long as you are able to get your requires memory density. The reference design has used 2 rank system to achieve their memory density.