I use the fronts & sides of my thighs. I start at the top on the first change of a month & work my way across & down. I change every 2 days as I used to get lumps after 3 days & the scars would take weeks to go. I change legs each month. Therefore I use 30 areas every 2 months, but of course, I doubt I hit exactly the same place twice very often.
I keep post it notes in my log book to remind me to change, I have a terrible memory for routine stuff.
I hate using my tummy, the set would always catch on my waistband & was uncomfortable, plus I don't have much flesh on my tummy which made site numbers limited.
I have found a great way of dealing with cannula marks/scars though. I'd heard that lavender oil was great for burns & gave it a try. It's fantastic on cannula marks too. I just rub a bit each night into both thighs & the marks are gone in a day or so. (essential oil bottles will always say not to apply undiluted to the skin, so you make your own risk assessment, but there's plenty of available evidence that lavender oil is safe)