How many attachments and how many years can I store in quickbooks? What is the maximum file size per attachment and total overall size available?

Afaik, there's no limit to the parameters you're asking about. Afaik, for Singaporean companies, financial records must be kept for at least 5 years from the end of the financial year when the relevant transactions are made. You can open a new blank account and purchase a third-party service to migrate data with attachments if needed to keep your data size small.