1998. And that's only because I wasn't able to port the number to a new carrier. I've ported my current number a couple of times since then and will continue to do so if permitted.
Last changed in 2008, when I got my first iPhone and moved to AT&T. I decided not to port my number from Sprint because it used an overlay area code that nobody recognized.
I changed once because I inadvertently gave it out in a screenshot to someone on a dating site I wasn't sure about. Didn't want to chance it. Although in hindsight I wondered if it would have mattered, because my number has been bombarded with robocalls more frequently vs the old number. This was couple yrs ago. But changing numbers is a huge hassle because of how tied in it can be to some accounts that use the number for security.
On my 3rd number in 17/18 years.
Only changed from the last one because someone I wanted no further contact with had a habit of changing their number and contacting me.
I plan on keeping this one.
I have changed numbers once (that I recall) since I have had a cell phone. T-Mobile does a very good job completely blocking scammers and spammers. When I was with Verizon, several years ago, that wasn’t the case.
If you need to give out your number, use a burner number from Hushed or something similar.