You get that screen because you collected more weapons from that battle than could fit in your inventory, so you get taken to that screen to sell some to make room. I'm pretty sure you can also sell the ones you just picked up, too. If I remember right, you need to select enough weapons to get...
You have 25/20 weapons because you collected more weapons in the battle than you had space left for in your inventory. I'd suggest just hitting random buttons until something works. I don't remember being confused by the screen myself, but I also only hit it once so, I'm sorry I couldn't be more...
I'm pretty sure you just press +. I haven't been on that screen in forever since I usually take care of my weapons before they ever get that full. However, I'm certain it works the same way as in the smithy.