It should be 4B, instead of 2B.

The current reaction is 4B Chlorine + 2 Carbon Dust + 1 Rutile Dust -> 1B Titanium Tetrachloride + 2B Carbon Monoxide. By right the reaction is TiO₂ + 2C + 2Cl₂ -> 2CO + TiCl₄. Since 2B CO is made from 1B Oxygen and 1 Carbon Dust, only 1 Carbon and 1B Oxygen is reclaimed from the reaction - 2 each is the correct amount, so total 4B CO should be produced

This is a vanilla GTCE recipe and appears to be the same recipe even in v1.9.0.

To clarify, what I mean is you should probably report this inaccuracy to GTCE itself: https://github.com/GregTechCE/GregTech/issues

We can patch it on our end with CT for now though.

GTCE generally uses a specific rule for quantities of dusts and moles: the sum of the atoms in the compound is the number of dusts required to be considered 1 mol of the substance. Typically, 1 bucket of a fluid is treated as 1 mol.

This is observable in the EBF recipe using 1 bucket of TiCl4 and 2 Mg: the result is an ingot of Titanium and 6 MgCl2 dusts, which is 2 mol MgCl2.

It gets a little weird with some recipes though. Whatever the reason, Rutile is not subdivided in the usual manner so it's 1 dust = 1 mol. Carbon dioxide is also a bit unusual in that 2 buckets is considered 1 mol (as evidenced by the electrolyzer recipe).

In this case you are using 1 rutile dust, effectively 1 Ti and 2 O. The two carbon dusts would pair with the oxygen to result in 2 CO, so yes it should be 4 buckets of output to balance.