You can remove them if you prefer, though suckering (sprouts arising from the roots or trunk/branch base at the soil line) is a normal feature of this and several other native shrubs. Over time, as the original main branches on each plant mature, you might want to let some suckers grow and mature into new branches at that point, so the oldest can be cut down to reinvigorate the shrub using that younger wood. It's not necessary, but an aesthetic choice, since sometimes younger growth flowers and fruits best, and some winterberry cultivars (as well as the straight species) can mature fairly large for a shrub.

If you want to remove the suckers, just prune them down to just below the soil line, if you can access that without cutting into neighboring growth that you want to keep. If too dense to reach, just cut them off at ground level, though expect that with either method, suckering might resume a bit, at least during the first half of the growing season. (Most shrubs are done with active growth by midsummer or so, when they switch to preparing that growth for winter hardiness.)

Miri