Sort first, then estimate the remaining load
Furniture, shelving and boxed belongings take up space differently. Describe the largest pieces and whether they can be safely taken apart. Start by comparing a 15-yard container for a smaller cleanout, a 20-yard for a larger mix, or a 30-yard for bulky material. Those are planning options, not guarantees that a particular house or number of rooms will fit.
Work through a garage cleanout