Which backups does Time Machine keep or delete?
For hourly backups Timemachine does indeed only keep the "first" backup of the day. This  is a "Daily" backup.

Backups are automatically deleted after a backup is performed and the storage of old backups is set according to this schedule.

Daily backups are deleted after a month except for the first of each week which is a "weekly" backup.

Weekly backups are deleted as space is required, then one or more of the oldest "Weeklies" will be deleted.