Each key block range (i.e. each group,range pair) contains a very large number of key blocks (over 4 billion). In normal usage, 256 keys are allocated from each key block, so the number of keys available from each range allocation is very large. Furthermore, each business object generally uses a separate key set (or collection of key sets), and so effectively has a separate "key counter" for use by its entities. This in turn means that keys are used up at an even slower rate than might otherwise be the case. Therefore, it is unlikely that key block range allocations will be exhausted in normal usage.
Nonetheless, to cater for the possibility of key block range exhaustion, a system can be configured with multiple key block ranges. Furthermore, these additional key block ranges can be added later if necessary - i.e. they can be added even after the system has first been started. If a system is configured with multiple key block ranges, it will transparently start using key blocks in the additional ranges when all key blocks in the original range have been used up.
Note that any additional key block range allocations must also be unique to the system - that is, they cannot be used on other systems within the system landscape. Also, as already noted, after a system has been configured with a particular key block range, it is not possible to remove that key block range from the system.
As an example, a production system could start with the allocation group 3, range 2. Then, later on, an additional allocation group 3, range 44 could be added if required. If all Key Blocks in the allocation group 3, range 2 are then used up, the system will start using keys from key blocks in the allocation group 3, range 44.
One point to note is that a log message will be output when a key set on the system is approaching the end of the range allocated to it. In the default system configuration, this message is output when the key set has 100,000,000 (one hundred million) key blocks left. This should allow for adequate time to provision the system with another key block range allocation in advance of the existing allocation being exhausted.