How many Turn-on/offs works best?

You can have up to 50 of each (Prefs & TOTOs), not 50 altogether.

Likes/Dislikes on can be gained through gameplay (Hobbies, Music, Characteristics and Conversation can anyway), so I like to give/randomise a few (1-3 in each category) and leave space for character development via gameplay. I play Mixed Music station to develop music taste in game rather than adding in CAS unless it's a big part of their character (such as a clubber loving DJ Music).

TOTOs can only be added to played sims manually. You could do this all at once, or add to to them/remove them as you play. With only a few you will have Neutral attraction to most sims, and Good/Bad with a few, so the dating pool will be wide.

I would start out with "dealbreaker" TOTOs that your sim needs in their partner. Some starting TOTOs would be things like working hours (Freelancer vs. Workaholic etc) or skills that compliment traits/prefs (a Foodie that Dislikes Cooking really wants a partner who cooks).  As you date various sims you can add Turn-Ons that you liked about that sim (they were Funny!) and Turn-Offs that you didn't (the last 2 dates went horribly, and they were both Cerebral sims). This mirrors how we figure out what we want IRL. More TOTOs will narrow the field, but you can always reassess if you go too far and remove an ick/boon from your list if you decide you can live with(out) it.

Also remember that sims' skills increase with time and traits can be gained and swapped out, so partners may lose/gain Attraction to your sim as you live your lives and grow together/apart, the same way platonic Compatibility can also change as traits are revealed.