The honest number is hard to come by because no lender publishes enforcement data and most investors who have had a loan called do not broadcast it. What does exist is anecdotal and directional: practitioners who have done hundreds of subject-to deals report it happening, but almost always when something made the lender look twice, not simply because a transfer occurred.
The triggers people report most consistently are: the property showing up in public records with a new owner name while the loan statement still goes to the old address, insurance changing to a landlord policy with a different named insured, and the seller contacting the lender directly after the relationship with the buyer soured. That last one is the most common cause I see cited. The lender does not usually go hunting. Something lands on their desk and gives them a reason to act.
Payments arriving on time and nothing changing on the account surface are the conditions under which lenders historically leave these loans alone. That is not a guarantee, and your attorney is right that the clause is real. The guide for this strategy puts it plainly: the due-on-sale risk is navigable for knowledgeable operators who maintain a clear contingency plan, meaning you need a realistic path to refinance or sell if the note gets called. At 4.1% in the current rate environment, your financing is genuinely valuable, so the contingency plan is worth building out in concrete terms now rather than treating it as a fallback you will figure out later.
A real estate attorney familiar with subject-to structures in Arizona is the right person to help you document the seller relationship and assess your specific exposure. I would not rely on general guidance here.
What does your contingency plan look like right now if the lender did call the note? That question will shape what preparation actually makes sense for your deal.