How big are the Royal Marines?

In 3 commando brigade there are 3 Commando regiments. (40 Commando, 42 Commando and 45 Commando) Each as about 1500 commandos.

Also the the fleet arm protection group (Commandos that work on ships) has about 1500 Commandos and the Air assult unit has a further 1000 Commandos.

Overall there is about 7500 Commandos