Not really true.

Just to be clear, it's 255 RJs with 51-76 seats and it doesn't matter how big the mainline fleet gets, that number is fixed.

The number of 71-76 seat RJs ("76" seat)is currently 153 and can go up based on the mainline fleet exceeding 767 airframes, but only so long as the aggregate number of 51-76 seat jets doesn't exceed 255.

Why is that important, because most "70" seat RJs are configured with 65 seats and more importantly, the number of "70" + "76" seat RJs is already scheduled to reach 255. Therefore, if the number of "76" seat RJs ever is allowed to exceed 153 (mainline above 767 aircraft) the company has to park a "70" seat jet for each "76" seat jet it adds.

It's important that we understand this subtle difference. 255 is a hard number. Let management trade 70 seaters for 76 seaters if they like, but let's not go in thinking that 255 is subject to change based on the size of the mainline fleet, it's not. Unless of course they are flown by a mainline pilots, in which case they can have as many as they like.