1. We need to count the elements from the list which show only one non-zero oxidation state.

Given elements: $\text{O, Cl, F, N, P, Sn, Tl, Na, Ti}$

2. Check each element:

(i) Oxygen, O

Oxygen commonly shows: $-2, -1, -\tfrac{1}{2}, +1, +2$ So it has more than one non-zero oxidation state.

(ii) Chlorine, Cl

Chlorine shows: $-1, +1, +3, +5, +7$ So it has more than one non-zero oxidation state.

(iii) Fluorine, F

Fluorine shows only: $-1$ It does not show positive oxidation state in compounds because of highest electronegativity. So this has only one non-zero oxidation state.

(iv) Nitrogen, N

Nitrogen shows many oxidation states: $-3 \text{ to } +5$ Hence not counted.

(v) Phosphorus, P

Phosphorus shows: $-3, +3, +5$ Hence not counted.

(vi) Tin, Sn

Tin shows: $+2, +4$ Hence not counted.

(vii) Thallium, Tl

Thallium shows: $+1, +3$ Hence not counted.

(viii) Sodium, Na

Sodium shows only: $+1$ So this has only one non-zero oxidation state.

(ix) Titanium, Ti

Titanium shows: $+2, +3, +4$ Hence not counted.

3. Therefore, the elements showing only one non-zero oxidation state are: $\text{F and Na}$

So, total number is: $2$