A single Failover server can monitor multiple AirWave setups. The licensing and sizing should be the size of the largest AirWave setup. Just know that if there's a failure, that failure needs to be addressed otherwise there's no additional failover coverage.
For example, if you have 3 AirWave servers: 2500, 3000, 4000. You'd want to size the failover to handle the 4000 size. This would mean it'd handle the 4000, but it'd also handle 2500 and 3000 size if they failed.