The name Thousand Sons is literally the name of the legion and, as far as i know, no reflection on the numbers. As they are a pre-heresy legion, they could easily have as many as 10,000 members.. if there is only a thousand
Technically they are already dead due to the Rubric, mearly being automated suits of power armour. I'm not actually sure for the regular troops, as due to the mutation problems its may be hard to induct humans into the ranks of the Thousand Sons, unless they had the Rubric applied to them. And I presume that they still recruit the Sorceror's as normal, but again I'm not too sure at this stage. SO they may recruit, or may not recruit.what happens when they are killed, do they recruit more?