According to Minnesota Dept. of Health:
A normal level of oxygen is usually at least 95% or higher. Some people with chronic lung disease or
sleep apnea can have normal levels of around 90%. The SpO2 reading on a pulse oximeter shows the
percentage of oxygen in someone’s blood.
The instructions that came with my home-use pulse oximeter (in German) roughly concur: 94-99% "normal". 90-93 "consult an MD", <90% "call an ambulance". It does also come with a table for compensating for (earth's) elevation too, as that affects O2 concentration. Basically below 1500m altitude 90 is the safety limit, but at 5800m for example that decreases to 80% and to 70% at 7500m. (It even comes with a citation for this part: Hackett PH, Roach RC, "High-Altitude Medicine" in Auerbach PS (ed.) Wilderness Medicine, 3rd ed., Mosby, 1995)
From a more recent study on that (high-altitude angle), there's actually a fairly substantial spread in how individual SpO2 responds to altitude