It depends. If we have an EMB-145EP, then yes, it may be weight restricted to 47-49 customers. However, if you have an EMB-145LR or XR, then likely you can take 50 customers and a cockpit jumpseater. We have a much, much better weight and balance program than American Eagle. We have five zones, credits for galley carts, detailed cargo loading, and we don't often run into CG issues. I think our weight and balance program is one of the most labor intensive of any regional airline, yet also allows us to carry the most payload.
On BNA-IAH-TUS, I doubt you'll see anything other than an EMB-145LR or XR. I think the only time you might see an issue is if the weather is very bad in IAH (low visibility or thunderstorms) and the dispatcher has put a lot of extra fuel on top of an alternate.