In case of half rate, a cell can handle twice more calls than in full rate.
On Abis interface, one call will occupy half an Abis nibble instead of a full nibble.
Problem of capacity will occur on the BSC side, depending on your vendor. normally, the BSC board connected to the Abis’s (TSU in alcatel system) is only able to handle a certain amount of calls, regardless whether it is FR or HR. So if you double the number of calls on the Air interface, you need to double your capacity in TSU.
BSC with a more modern conception should be able to cope better with HR (cf. the latest generation “BSC Evolution” for Alcatel)