If you have 96 ng of plasmid then in theory you need 0.1U of enzyme to digest in in an hour (assuming the unit is defined as 1 hour). Practically speaking if you add 1-2U you should be fine.

The product insert that comes with the enzyme will have a standard protocol for how much to use.

Do you mean you have exactly 96 nanogram of plasmid OR do you have a plasmid at the concentration of 96 nanograms per microliter?