Databases are allocated among the devices in fragments called "disk pieces", where each disk piece is represented by one entry in master.dbo.sysusages. Each disk piece:
Represents a contiguous fragment of one device, up to the size of the device.
Is an even multiple of 256 logical pages.
One device may be divided among many different databases. Many fragments of one device may be apportioned to one single database as different disk pieces.
There is no practical limit on the number of disk pieces in one database, except that the Adaptive Server's configured memory must be large enough to accommodate its in-memory representation.
Because disk pieces are multiples of 256 logical pages, portions of odd-sized devices may remain unused. For example, if a device has 83 Mb and the server uses a 16k page size, 256 logical pages is 256 * 16k = 4 Mb. The final 3 Mb of that device will not be used by any database because it's too small to make a group of 256 logical pages.
The master device sets aside its first 8k bytes as a configuration area. Thus, to avoid any wasted space, a correctly-sized master device should be an even number of 256 logical pages *plus* 8 kb.