I keep seeing people refer to “a NURB” when they mean “a NURBS”. “NURBS” is the singular form (i stands for “Non-Uniform Rational B-Spline”).

If you want a plural form, how about “NURBSes”?

As an acronym, standard pluralization rules need not apply, so it could easily be treated as both, like fish. But as you say, there is no such thing as a singular NURB.

NURBSZ ?

Or you could finesse the issue by only using “NURBS” as an adjective, e.g.

singuler: “NURBS patch”, “NURBS curve”
plural: “NURBS patches”, “NURBS curves”