The number of disks you can use in a volume depends on the type of volume you're creating.
From what I vaguely remember, you cannot exceed 12 disks per volume for RAID5 and RAID6 with Synology. Don't quote me on this.
However, as you can see below, I've successfully created a single SHR1 volume with 24 drives.
"Hi, What is your Server Config., I just want to know as I am planning to build Highend NAS using AMD 990FX Chipset with 6 Drive. Planning for SHR1 / Raid 10. Please reply I will be waiting"