Time to walk to Platform 23 from NEX

I am arriving atTokyo Station on the Narita Express at 9:20am, I need to get to the Shinkansen on Platform 23 to travel to Nagano, this train leaves at 9:32am

Is 12 minutes enough time to walk to Platform 23?
I am roughly aware of how to get there by gong up the escalators to BF1 and following the green shinkansen signs.

Yes, 12 minutes is enough. I've done it in five (running), but normally I would allow 10-minutes for it. A little tight.

Sounds tight. Assuming you come from the airport I wouldnft book the Shinkansen nor the NEX. Once you are at Narita take the next NEX. That might be the one you are targeting now or before /after depending on your actual arrival.
You can buy the Shinkansen ticket at Narita and buy one that gives you a little bit more time in Tokyo. 12 min May be doable but thatfs for someone who hasnft just completed international travel, has luggage and critically doesnt know Tokyo station. You donft want to start your holiday with stress. And there are many Shinkansen to Nagano. So no need to stress.

The Ekitan website tells us that even a fast walker should take the 9:44 Asama if the person was to arrive Tokyo at 9:20 on NEX. The 9:44 Asama arrives at Nagano Station at 11:33.

Another option is to take Skyliner 4 arriving Keisei-Ueno at 09:16, and walk to JR Ueno to catch the 09:38 Hakutaka 557 arriving Nagano at 11:03.

I hope it helps.