I have several Ashford wheels and at the last count 14 single drive bobbins, about 7 double drive I think.
However, I do lots of spinning-related activities, plus attend several spinning groups, including my own, and do events where we 'give people a go', so I may need spare bobbins for things besides my own main projects.
Thus, for people like me, the mantra is 'buy a new bobbin at every event where they sell them', and 'you can never have too many bobbins'!
Other factors affect the answer too. Some people like to spin all the singles for the project, then ply 1st and last, 2nd and penultimate, etc, to even out any changes in spinning as the project progressed. Others (like me) spin two (or three) bobbins, ply them, spin two more, and so on. Then again, you can wind off onto storage bobbins if you're keeping singles for some time.
I do agree with FW that too many bobbins encourages 'flitting'