comparing to one instance,what do multiple instances require about the system hardware?
more memory? higher cpu frequecy? or more cpu cores?
thank you!

The server has finite resources (memory and CPU). Each instance needs to utilize those resources. More than one instance means they have to share. Allocating the resources optimally can be a major project. You should use multiple instance only it it is required.

Andy

Generally speaking, memory is the limiting factor when there are multiple databases (in same instance or in separate instances) on the same server. If you have two databases, and trying to decide whether to put them in the same instance or two different instances, then there usually is not a big difference in the amount of memory required for either option, since the bulk of the memory requirements are DB2 bufferpools for each database.

If you have two databases, there is very little difference in CPU requirements whether you put them in one instance or have two separate instances.

probably still better have databases in different instances. then you can have different dbm cfg parameters and if you bring down one instance another instance (and database) still remains available.