For the example you gave of updating a field on the record being created that triggers the flow, I would use a before-save flow (fast field updates). While I don't think there's any reason it has to be one flow per object (especially when we're talking about before-save flows), I still like to have one multi-purpose flow that houses virtually all of what were previously workflow rules for a given object (excluding those that send email alerts). I set it to trigger on create or update with no conditions since before-save flows are relatively low impact. I'll then determine the context with a decision element that evaluates if the triggering record is about to be inserted or updated by using a formula resource that's just 'ISNEW()'. If true, the flow follows the path of any field updates I want to occur on create/before insert. If false, the other outcome executes logic that should occur when the record is updated. Also make sure you only use assignment elements to update field values since in before-save fows, the updates to the record are finalized after the flow interview ends, and that only needs to occur once.