Do you need two DATASUCKER'S if you want to lower the strength of a piece of ice -2 ? Or will

1 DATASUCKER with multiple counters work?

One data sucker with multiple counters will work.

Unlike Imp, there is no limit to the number of hosted counters that can be spent from Datasucker (Imp is one per turn).

You'll often see runners make repeated attacks against an undefended Archives simply to charge Datasuckers before running a server they actually want to access.

Edit: it's even more common if the runner is also using Desperado, as they're now charging Datasucker AND generating a credit with one click.

All hail datasucker and desperado!

Seriously though, desperado is the only console I play these days unless I'm playing an anarch virus deck. Hell even my Kate, Kit, and CT decks use desperado and (often, though not always) datasuckers.

Being able to threaten all servers forces the corp to spread their ICE thin and forces them to use credits to defend everywhere instead of concentrating on one spot.