Initially, each BIT share will represent 0.1 bitcoins, but the trust will not generate any income and will regularly sell/distribute bitcoins to pay for on-going expenses, so the amount of bitcoin represented by each share will gradually decline over time.
It says it will do that only to cover ongoing expenses, not to make room for new investors. If more than 2mil wants in, they will have to buy more, which you said they wouldn't.
If I'm wrong, by all means fill me in.