I start off with 2 weapons per trap. This lets me deploy weapon traps fairly quickly. I'll install them where needed (often replacing my starting cage traps). Then once I have my trapped hallway set up to my liking, I'll create more weapons for the traps or higher quality weapons (ie, replacing large serrated iron discs with large steel serrated discs), and then upgrade/replace the current trap line in small blocks. I repeat that until I have a sufficently weapon trapped corridor that can handle large sieges without me.
I settled on this rather then waiting on the necessary "final" weapon configuration as it allows my dwarves to stay safe and sound WHILE they go from the quickly in place cage traps to weapon traps (backed by cages) to strong cage weapon traps. I find it best to have something to stop the attacks and ambushes and uninvited guests in place as soon as possible. I've had goblin ambushers in DF on the first day of year 2 of the fortress, and now in 0.31.16, you can get kobold thieves in the first month of a new fortress. Defense will be needed and sometimes very quickly...
The more traps you have, the bigger a mass of enemy it can handle, as weapon traps ALWAYS jam, even with masterwork mechanisms loaded with 10 masterwork large steel serrated discs. Design accordingly...