The Magic Number measures how efficiently sales and marketing spending converts into new ARR. The standard formula is net new ARR in the current quarter (annualized) divided by S&M spend in the prior quarter. A Magic Number above 1.0 means the company is generating more than $1 of ARR for each $1 of S&M; above 0.75 is generally healthy.
It's similar in spirit to the burn multiple but isolates sales efficiency rather than total capital efficiency. Companies use both to triangulate where to invest more or pull back.