Professional I always keep 4 in my bag.
I have had days recently where I'll have two jobs in one day and I Shoot photos and video on every job. Unless the realtor says I only need photos.
I've actually just purchased 2 additional slightly used batteries to have just in case.
On a windy day shoot a large property with photos and video, two rounds I can easily burn 2 batteries in one job.
I have purchased a couple of new batteries on eBay without pen package for $80.
If you're making money flying it just makes sense.
If you're not, 2-3 should be fine.