The term is usually used by M&A (mergers and acquisitions) firms, investment bankers, and venture capitalists.
There’s no specific way to calculate deal flow. The term is often used in a qualitative rather than a quantitative way. For instance, “deal flow is good” or “deal flow is better than last year.”
The concept of deal flow can also incorporate the quality of the proposals. Being offered lots of low-quality potential deals doesn’t make for a good deal flow.