To address this question, we'll consider each element listed to determine if it exhibits only one non-zero oxidation state. This requires looking into common oxidation states that each of these elements can attain.

Oxygen (O): Oxygen typically shows an oxidation state of -2, except in peroxides where it has -1 and in compounds like OF2 (oxygen difluoride) where it exhibits +2. Therefore, it shows more than one non-zero oxidation state.

Chlorine (Cl): Chlorine commonly exhibits oxidation states of -1, +1, +3, +5, and +7 depending on the compound it is part of. Thus, it shows multiple non-zero oxidation states.

Fluorine (F): Fluorine is the most electronegative element and almost exclusively exhibits an oxidation state of -1 in all of its compounds. Hence, it shows only one non-zero oxidation state.

Nitrogen (N): Nitrogen can exhibit a variety of oxidation states ranging from -3 to +5. Therefore, nitrogen too shows multiple non-zero oxidation states.

Phosphorus (P): Phosphorus commonly exhibits oxidation states of -3, +3, and +5. So, phosphorus also has multiple oxidation states.

Tin (Sn): Tin typically shows two common oxidation states: +2 (Sn(II)) and +4 (Sn(IV)). Therefore, it has more than one non-zero oxidation state.

Thallium (Tl): Thallium primarily shows +1 and occasionally +3 oxidation states. Hence, thallium also exhibits more than one non-zero oxidation state.

Sodium (Na): Sodium almost always exhibits a +1 oxidation state in its compounds. Therefore, it shows only one non-zero oxidation state.

Titanium (Ti): Titanium can exhibit multiple oxidation states; common ones include +2, +3, and +4. Therefore, it has multiple non-zero oxidation states.

From the elements listed: Fluorine (F) and Sodium (Na) are the only ones that show only one non-zero oxidation state. Thus, among the given elements, 2 of them (F and Na) show just one non-zero oxidation state.