How many moles of CO are produced when 1.9 mol of C reacts?

How about a reaction?

Are you suggesting that burning carbon produces CO?

A common misconception is that burning carbon with a "limited" supply of oxygen will produce CO, while burning carbon with an excess of oxygen will produce CO2. That isn't entirely accurate.

If there is insufficient amount of oxygen to convert all of the carbon to CO2 (an excess of O2) then there will be a mixture of CO and CO2 with no way of determining the ratio unless a measurement is made.

Assuming that all of the carbon reacts then (n) moles of CO will form along with (1.9-n) moles of CO2.

========= Follow up =========

The reaction in question is....
5C(s) + 2SO2(g) --> CS2(l) + 4CO(g)
1.9 mol ................ ..................? mol

Get used to using the unit-factor method.
1.90 mol C x (4 mol CO / 5 mol C) = 1.52 mol CO .............. round to two significant digits: 1.5 mol CO

Using the equation given:
5C(s)+2SO2(g)→CS2(l)+4CO(g)
5mol C produce 4 mol CO
1.9mol C will produce 1.9*4/5 =1.52 mol CO
To 2 significant digits: 1.5mol CO2 produced.

2CO(g) + O₂(g) → 2CO₂(g)
2 moles of CO reacts with 1 mole of O₂ to give 2 moles of CO₂.

Number of moles of CO reacted = (1.9 mol) × 2 = 3.8 mol
Number of moles of CO₂ produced = (1.9 mol) × 2 = 3.8 mol

2C + O2 = 2CO
Hence
by equivalence 1.9 moles of C produces 1.9 moles of CO