I appreciate your time in reaching out to us in the Community, socal-matt1983-i.
Let me share some info about your concern.
At the moment, you can only track miles for one vehicle in QuickBooks Self-Employed. We're working with TurboTax to update this feature, so you can track multiple vehicles in the future.
I'd also recommend sending a request to our Product Development Team by following these steps:
In your QuickBooks Self-Employed account, go to Assistant.
Type Submit feedback. Choose the Feature request option from the prompts.