SpaceX Could Soon Pull Off the Biggest IPO Ever -- but Investors Should Watch This One Critical Number

According to the Chinese zodiac, 2026 is the "year of the fire horse." For U.S. investors, though, 2026 probably deserves the title of "year of the mega-IPO." Artificial intelligence (AI) leaders Anthropic and OpenAI are expected to conduct IPOs with valuations in the ballpark of $1 trillion. However, the most highly anticipated public listing is unquestionably SpaceX.

When Elon Musk merged his AI start-up, xAI, with SpaceX in February 2026, the space technology company was valued at roughly $1.25 trillion. SpaceX's IPO valuation is estimated at around $1.75 trillion, making it the largest IPO ever -- by far.

Such a lofty market cap for a new publicly traded company will deservedly attract much attention. However, there's another critical number that investors should watch with SpaceX.

Image source: Getty Images.

Looking under the hood at SpaceX's business

Before we get to that important number, it's helpful to first examine SpaceX's business model. Actually, the company has multiple business models.

SpaceX's launch business receives the most public attention. Many people have watched the Falcon 9 rockets take off and later land after soaring into space. SpaceX charges a hefty fee to launch satellites and take crew and cargo to the International Space Station. In the not-too-distant future, the company will generate revenue with its Starship reusable spacecraft.

As previously mentioned, Musk merged xAI with SpaceX earlier this year. xAI makes money from subscriptions for its Grok large language model (LLM). Former xAI CFO Jonathan Shulkin told investors that the AI unit is on track to be profitable by 2028.

But the biggest moneymaker for SpaceX right now is its satellite internet service, Starlink. This business offers high-speed internet to residential and business customers using its constellation of more than 10,200 satellites.

Starlink raked in revenue of $11.3 billion last year, according to The Information. That amount is roughly 61% of SpaceX's total 2025 revenue. Analysts project that Starlink will generate around $20 billion of revenue in 2026.

SpaceX's critical number

Since Starlink is SpaceX's largest cash cow, it stands to reason that the company's most critical number would be related to its satellite internet services business. There are several important metrics to watch with Starlink.

For example, subscriber growth reveals how strong the demand is for Starlink's services. The unit reported over 9 million customers in 2025, of which more than 4.6 million were added last year. On Feb. 13, 2026, Starlink posted on X (formerly Twitter) that it "is connecting more than 10M active customers with high-speed internet across 160 countries, territories and many other markets."

However, subscriber growth isn't the most important number for Starlink. For one thing, some subscribers don't make the company nearly as much money as others. Also, Amazon (NASDAQ: AMZN) plans to begin offering satellite internet services with its Amazon Leo unit later this year. It's possible Starlink could face pricing pressure from new competitors.

Because of these factors, I think the most critical number to watch with SpaceX following its IPO is Starlink's average revenue per user (ARPU). In particular, watch the ARPU trend. This metric will reveal whether Starlink is relying more on rural residential subscribers or on higher-margin business customers, such as cruise operators. If ARPU grows robustly, SpaceX is on the right track.

The ultimate number

To be sure, Starlink's ARPU is only the most important metric to monitor initially. Ultimately, profits matter most for a company. Sooner or later, SpaceX will need to generate significant earnings to justify a valuation of $1.75 trillion.

However, rapidly growing ARPU would be a positive indicator that profitability is realistically attainable for the space stock. If ARPU growth is weak, SpaceX could join the list of other high-profile IPO stocks that crashed and burned.

Keith Speights has positions in Amazon. The Motley Fool has positions in and recommends Amazon. The Motley Fool has a disclosure policy.