It might be too much. I'd only ever go max 10. Ten can usually do the smashing job you need done, plus, any more than that and you might have trouble deploying them wholly within 3", outside of 3" of enemy units. I'd also pose that Fanatics have had their market value continually decrease as more new battletomes include "always strikes first" "strikes last" and "impact damage" units---meaning those units of bloodcrushers, gore gruntas, terrorgheists, every Slaanesh hero, or whatever will be able to annihilate your little dudes before they even get a chance to swing, or force you to delay their deployment, which nullifies a lot of their initial value.
The biggest thing about Fanatics is they're all or nothing. They can interrupt a big charge, wipe out a big scary unit, and laugh all the way to the bank. But just as often, they'll be countered by some ability or smart movement by the opponent...and that's 140-280 points that ended up doing nothing. Even if everything goes perfectly, you might just roll poorly on their attacks, leaving them helpless. To be honest, I really think Loonsmashas should be more like 120 and Sporesplattas 100.