It’s good practice to remember that the location of your event is very important to the frequency range of the hire radios you require!
Planning an indoor event? Ultra High Frequency (UHF) offers the best penetration versus man-made structures. The narrow wavelength finds it much easier to transverse concrete, brick and wiring than Very High Frequency (VHF) but at a price that it will not carry as far. As UHF radios are better suited for built up areas, they are very popular in cities. This increases the likelihood of radio systems being used in close proximity potentially on the same frequencies. As a result UHF band radios are limited to a maximum power of 4 watts.
As far as communication range would be concerned, you should be able to cover several hundred meters but this is entirely dependant on location. If the location contains substantially thick walls, elevator shafts and a lot of electrical cabling then reception will be severely reduced to potentially tens of meters. Largely, this can be overcome by the use of a Radio Repeater which can boost the signal (depending on licence limitations) from 10 watts to 40 watts.
Planning an outdoor event? VHF comes into its own here. The wavelength of VHF is longer than UHF and as a result carries further. This is further boosted as VHF band radios are permitted to use 5 watts as supposed to UHF 4 watts limit. In a country or park location with minimal structures blocking line-of-sight between the two users a distance of several miles can be achieved. This can be further increased by utilising a Radio Repeater system and desktop or in-car mobile radios. With a repeater mast at an elevated location and the signal boosted (depending on licence limitations) by 10 watts to 40 watts this can greatly increase the area of coverage. Still, black-spots are to be expected in areas that dip out of line-of-sight or substantial structures block the signals path.
The best way to check suitable coverage would be a site test. This is especially important where communications are critical for human safety. A site test will identify the equipment required, the most suitable location if a Radio Repeater is needed and identify any black-spots.