The basic guideline is that if you are observing an individual organism over different days, you should make them separate observations rather than add new photos to the first observation. At this time, you can link observations together by using comments, the observation description section, or observation fields. But, this is an area of iNaturalist that could use some improvement.
There are several existing topics about this subject you might want to take a look at:
https://forum.inaturalist.org/t/uploading-observations-of-the-same-individual-over-time/2956
https://forum.inaturalist.org/t/link-observations/1367
https://forum.inaturalist.org/t/using-the-field-similar-observation-set-for-linking-observations-of-lepidoptera-when-raising-on/1018