How many ATP molecules are produced thorough ETS, out of 38 molecules produced after complete oxidation of one glucose molecule?

During the complete oxidation of one glucose molecule, a total of $38$ ATP molecules are produced.

Substrate-level phosphorylation (SLP) directly yields ATP without the involvement of the Electron Transport System (ETS). The ATP produced via SLP is:
Glycolysis: $2$ ATP
Krebs cycle: $2$ ATP (one per turn, two turns per glucose)
Total ATP from SLP = $4$ ATP.

The remaining ATP molecules are generated through oxidative phosphorylation via the ETS from reduced coenzymes (NADH and FADH$_2$).
Total ATP from ETS = Total ATP $-$ ATP from SLP
Total ATP from ETS = $38 - 4 = 34$ ATP.

Alternatively, calculating from reduced coenzymes:
Glycolysis: $2$ NADH $\rightarrow 6$ ATP
Link reaction: $2$ NADH $\rightarrow 6$ ATP
Krebs cycle: $6$ NADH $\rightarrow 18$ ATP and $2$ FADH$_2$ $\rightarrow 4$ ATP
Total ATP from ETS = $6 + 6 + 18 + 4 = 34$ ATP.

Answer: $34$