Say I have Multiples of Daedalus or Neo-Daedalus (or multiples of both at the same time) on the field. Do each of them require me to send a separate Umi/ALO to the graveyard to activate their effect, or can I just use one to activate the effects of all Daedalus/Neo-Daedalus' on the field?

Also, if I'm able to just sacc one Umi/ALO to activate multiple Daedalus/Neo-Daedalus' at once, what would happen? Would their effect destroy each other?

Each card requires its own cost. You must use one "Umi" for each "Levia-Dragon - Daedalus" / "Ocean Dragon Lord - Neo-Daedalus".

Yep. I agree with Tyrin Claw.