No it should be around 1200-1300 range compared to all other marine regiments - like i said it got mauled
taking an atoll but it should be getting reinforcements to bring it back up to full strength or close to it
OK, I see what you're looking at. You're talking about the troop count, not the number of squads.
Moving to a larger base may help, but you need squads in the pool to get reinforcements. If you've taken heavy losses, chances are the pool is depleted. If that's the case, all you can do is wait until more squads are produced. Move the regiment to somewhere safe, set it to train, and leave it to rebuild.
If you want to check the pool, go to the Intelligence screen and click the button for Industry/Troops/Resource Pool. You can use the filters to only show US Marines squads. USMC Rifle Squads have a build rate of 54, so if the pool is empty, it will take 2 months to build enough to restore the regiment (a full strength USMC regiment has 102 Rifle Squads). That's assuming no other units are taking reinforcements (a pretty chancy assumption).
There are other potential problems depending on the date. If you're into 1943, you're producing the USMC Rifle Squad 43, but if you have Upgrades set to Off on the unit, it may be looking for the old squads. Make sure the squads in the regiment match up with what's being produced. The easiest way to do that is just make sure Upgrades are On.