Longstrikes are 180pt for 3, and only 6 wounds. They are a fragile and expensive unit. If they ever take damage you are in a seriously bad position, and sometimes you just can't stop that from happening, like when facing Skyfires.
From ~1.5 years of playtesting and tournament experience with Longstrikes (first with Aetherstrike, now with Anvils) I have found 9x Longstrikes is the perfect amount - it deals enough to kill almost any target with a double tap, but it doesn't overcommit to shooting and gives you enough points to bring the right support units. You can bring 12, but the extra 3 shots per volley is often overkill and increases the footprint of your unit significantly.
You never want to fully commit to shooting with any army in AOS 2 because there are some hard counters (Sylvaneth woods, Plaguebringers, Fulminators) and specific Realm rules can completely neuter you. A mix of melee and shooting is preferable since it gives you tactical flexibility.
Furthermore, while it is tempting to go all-in on shooting with the Anvils command ability you don't want to put all your eggs in one basket. A dual threat approach of Longstrikes and Evocators (or a similar melee powerhouse) makes it much more difficult for your opponent to counter since they have to wipe both units off the table to deny your insane damage output.
When building Anvil lists you should try to strike the right balance of long range shooting power, a melee hammer unit, and the right support units. It just so happens that 9 Longstrikes is the right amount to optimize all these factors.