Here's a study that looked at this: https://pmc.ncbi.nlm.nih.gov/articles/PMC10159647/#:~:text=Prevalence%20of%20ONJ%20among%20Oral,years%20of%20oral%20bisphosphonate%20treatment.

To summarize the results:
9 patients out of 8,572 on bisphosphonates developed ONJ.
5 developed ONJ spontaneously without any dental procedure or dental symptoms.
4 developed ONJ after tooth extraction.
BP durations ranged from 2.6 to 6 years.
4 were healed or almost healed when they completed the survey; 5 were not and in one case was worsening.

There were also 10 more cases where it says they did not meet the specific criteria of ONJ (refers to them as "ONJ-like").

The study discussion at the end is very interesting. Generally speaking, they think the incidence could be higher than what it is and refer to studies conducted abroad that have reached conclusions where the incidence is lower and higher.

I would agree that it makes sense that the incidence is low because many don't need dental procedures but if you do, your risk jumps.