Just one?
If you are tuned into your choice of substrate, you can pot it so that the top radial roots will be okay and the tap root will drown.
You can also just bend the tap root sideways when you plant it.
And you can cut it before you pot it - basically, the younger it is when you do it, the better.
If you had 3. 6, 9, 12, ... you could do a little experiment and make yourself be the world's expert on this question (maybe you don't care about the world and just wanna know, as is so with me.)
btw, I get some of my jollies by digging and potting stuff that pops up in the landscape around my house. Among these things were a couple of walnuts evidently planted by the neighborhood squirrels (walnuts are common Christmas time consumables around here and people sometimes toss them out to get the local wildlife through the winter). I put them into small (i.e., 2-inch dia) pots. What I saw was that they tend to make nicely tapering trunks with short internodes on this scale. Leaf sized was smaller than a full sized tree, but comically oversized for a mame/mini bonsai. I also treated the compound leaves as 'select-a-size', cutting them to just a few leaflets (which tends to induce back-budding), but I could never get to anything I thought attractive --> I had a good time with them, they died, I moved on. On the other hand, I've done ridiculous stuff with horse chestnuts and had a ball as their compound leaves radically reduce just by keeping the tree small.
So, carry on and enjoy that walnut!