The max you can currently get is 138 SDI with just gear. For a list of all the things you'd need and an updated list of items with
SDI on them. This is not including the 5 SDI from a
City Trade Deal or the added SDI from
Arcane Empowerment (40 SDI with L6 Focus) or
Reaper Form (16 SDI). Then there is the bard buffs
Inspire which can give you a max of 22 more SDI with a 4x pure bard. If you are a Gargoyle and you use the racial ability called
Berserk you can get another 12 SDI. Finally there is the food buff you can get with
Summer Dragonfish Pie which is another 5 SDI. So with all these things added up you'd end up with 238 ( 138 gear + 5 Trade buff + 40 AE + 16 RF + 22 Inspire + 12 Berserk + 5 food) SDI as the theoretical maximum.
That is all for just the highest SDI number you can get, but that doesn't mean you can't get higher damage with the use of other debuffing spells or items that would increase your damage but not increase your SDI number.