Where is the rule that make the bonus from multiple wizards staff stack. I could not find it?
Page 337 of the BBB 'Extremely Common Magic Items' basically tells you that you can have as many *-items as you can carry, the limit is simply points you can spend on them.
Now, while there isn't a specific rule that says that multiple Wizards Staffs' stack, there is the generic rule of special rules stacking.
I was hesitant about this being an actual thing at first, but then I read the rule for the Wizard's Familiar right above the Wizards Staff in the Empire magic items section: It is an extremely common magic item(*) that grants +1 to dispel - but it's also 0-1 per Wizard, meaning that if the +1 modifier to dispell didn't stack, there woudn't be any point to having a 0-1 limitation on it, a limitation that the Wizard's Staff lacks.
Honestly, if someone wants to spend 100 points on 5 Wizard Staffs' in a game against me to get +5 to cast on Assailments and Magic Missiles I'd allow it, even if the intent is a bit unclear.
Why? Because the mental image of how that wizard looks on the battlefield with all those staffs is hilarious + I'd absolutely laugh my ass off when you roll spells for your Wizard and don't get any Assailment or Magic Missiles. Maybe you should've gotten that Lore Familar instead huh?
(Yes, I know some lores have Assailments and/or Magic Missiles as Signature spells, don't ruin the moment...)