based on the reaction
2 HCl + Na2CO3 --> H2O + 2 NaCl
OR
2H3O+ + CO32- --> H2O
mole ratio is
2HCl ? Na2CO3
assuming all the Na2CO3 reacted
assuming titration end-point was reached accurately
we can conclude the reaction took place in exact mole ratio as described above.
given the moles of HCl that reacted, you can find the moles of Na2CO3 that reacted, based on mole ratio.
since mass of mixture = mass of Na2CO3 + mass of NaCl
mass of NaCl = mass of mixture - mass of Na2CO3
it cant get any simpler, i hope