Throttle bodies have nothing to do with fuel injection. They are simply housings for the butterfly air restrictors which we call throttles.
One per cylinder is common on race engines for razor sharp response. One per engine or one per cylinder bank is more usual on road cars.
Fuel injection is most commonly now of the one per cylinder type, although there have been single injector cars where the injector is mounted in the [only] throttle body.
Generally reserved for the cheaper end models, and also very common in 1991-1992 with the widespread adoption of catalytic converters in the UK whose A/F requirements could not be metered accurately enough with a carburettor so manufacturers retrofitted a throttle body injection setup in place of the carburettor.
NB: Diesel engines don't have throttles.