${\mathrm{SiCl}}_4\stackrel{{\mathrm{H}}_2\mathrm{O}}{⟶}X\stackrel{1000}{⟶}Y$

In the above reaction, no. of oxygen atom(s) present in X and Y are a and b. (a+b) is

On hydrolysis, all the bonds of SiCl_4​ are broken after which H⁺ forms bonds with Cl⁻ and OH⁻ forms bonds with Si⁴⁺, thus forming H₄​SiO_4​ and HCl.
On deep heating, H_4​SiO₄​ loses two water molecules to form SiO_2​

Hence a=4, b=2 ; a+b=4+2=6