The system includes 8 predefined invoice templates, which cannot be deleted (but can be used as a base to create your own templates).
These templates are always available when you select system default invoice template (Settings > Invoice settings > Default template), or default invoice for a customer (Customer editing form > Main customer settings > Default invoice template).
Examples of default invoice templates:
Invoice_v1
Invoice_v2
Invoice_v3
Invoice_v4
Invoice_v5
Invoice_v6
Invoice_v7
Invoice_v8