What is it? I think it's +6 Shuckle with assault vest in a sandstorm, but I'm not sure.

You are correct.

Shuckle with Maximum SpDef is 614. Since 6 stages multiplies this by 4, you now get 2456. Assault Vest multiplies this by 1.5, and this turns into 3684. Also, in Sandstorm, SpDef is multiplied by 50% (1.5). This turns your number into 5526!

Sources: Here Here (Shuckle's base stat) Here (Items, abilities and moves that increase SpDef)