I have a TS531P and I am considering to upgrade. I see no 5 bay NAS is on sale. I wonder why: 5 bays is useless or not reccomended? Or it is a a cost reduction in QNAP product lines? I am RAID5 with 4TB x 5 HDD so I am looking for a NAS with same capacity for backup. Even used marked (maybe another 531P)
Thanks
I am not sure why the 5 bays are no longer made.
But I think you could maybe consider the TS-632X or the TS-664.
Comparison: TS-664, TS-632X Compare products | QNAP
In any cases, make sure to follow the migration procedure (Direct system migration), to avoid losing data.
Your choices are all even numbers. 2, 4, 6, 8, 12, 16, etc.
Not sure why one would want a 5 bay. Sure it would allow you to fill everything for RAID6, but why not just have an extra spare bay then in a 6 bay NAS.
9 bay units have 5 x 3.5 and 4 x 2.5 bays
e.g.
I am mistaken about even bay numbers! 
Nothing wrong with it 
I have a TS-563.
QNAP has always been at the forefront of 5-bay design! While our 4-bay and 6-bay models provide the most complete RAID flexibility, we have taken the 5-bay concept to the ultimate level by introducing ‘5+4 hybrid storage’ models like the TS-932PX, TS-h973AX, and the upcoming TS-h966TX. We will continue to build innovative NAS models that combine performance and expandability for users in the future!