When you provision a Managed Metadata (Taxonomy) field in SharePoint, it creates a hidden field (of type Note) along with the main taxonomy field.
SharePoint for some reason doesnt delete that hidden field. So when you provision the field again with the same name, it simply increments the number for the hidden field. Lets say your field is called MyTax1. The hidden field will be named MyTax1_0. Now if MyTax1_0 already exists, SharePoint will create MyTax1_1 and so on.
How to delete these created Hidden fields on SharePoint Online tenant using CSOM or powershell?