Owners carry contingency for design changes. Contractors carry it for risk in their own work. It is different from an allowance, which the documents define for a specific item.
Example: a renovation contractor carries contingency for hidden conditions behind existing walls, because nobody can see them before demolition.
The right amount depends on how complete the drawings are and how much risk the contract puts on you. Do not use contingency to cover a poor takeoff. Find the unknowns, ask questions through an RFI, and exclude what you cannot price.