Companies that IPO generally have a certain amount of annual revenue, a certain sized customer base in particular sectors (e.g. SAAS), and have a certain valuation judged by market capitalization. Companies have to be a certain minimum size and must have a predictable business with stable metrics for public exchanges and markets to be willing to list and purchase shares in the company. This doesn't always require being profitable as of the IPO, but it typically requires a credible plan for achieving profitability (in the eyes of investors).

According to the 2017 IPO Report [1], citing SEC data, the median IPO offering size in 2016 was $95 million, and the median annual revenue was $66.5 million. To pick an example according to Crunchbase, Mulesoft raised $221 million in its 2017 IPO at a valuation of about $4 billion. They had a revenue in 2016 of $188 million with a year-over-year growth rate of 70%. For another example, Dropbox's 2018 IPO valued the company at $11 billion on revenues in 2017 of $1.1 billion.

[1] https://corpgov.law.harvard.edu/2017/05/25/2017-ipo-report/