As many as you need.
It varies depending on if you're fully incontinent, and bowell or bladder too. Plus it varies for what type of diaper you're using.
Changes per day isn't what you're really needing to look at anyways. What you need is COST per day. Ideally this should be under $5.00 or $6.00 per day.
If you use a cheap $0.50 diaper you'll change more (and probably leak more too), maybe 9 times per day. That's $4.50 so you're fine. And if you use an expensive $1.50 diaper, but only change 3 times per day then that's still only $4.50 and is still fine.
Of course, the more expensive the diaper, the more trustworthy and reliable they tend to be. You'll change less often which is more convenient, and yet also have less pee against your skin thanks to their superior absorption capabilities. Inserts and boosters (which add cost) will also not be necessary in most cases too.
Generally speaking, this is why so many here recommend premium diapers like northshore or confidry. And with these diapers, you'll typically find 3-4 changes per day end up being what you need. Once you add in severe bowell incontinence though, then extra changes throughout the day become necessary. Going with a slightly cheaper diaper offsets your total cost and keepsthe costs in balance.