Rules for what weapons a model may take will be either in the unit's army list entry or in the appropriate wargear/items list.
Generally they specify that a model may replace
x with
y so it's a matter of replacing certain weapons (generally resulting in as many weapons allowed as they ordinarily come with, though replaced with something else) although sometimes it's phrased that a model may simply take a weapon, in which case it is additional to other weapons.
But yes, there's no general, standard or set limit to the number of weapons a model can take beyond its specific options.
So to answer your question, simply follow what the army list entry says about what options a Nob can take, and follow those. If it refers you to the ranged or melee weapons list then follow the rules there