How do I track a job or project number against asset checkouts

You can create a custom field called project number and enable the following options:

1. Display at Checkout
2. Only record with Checkout event. Do not update the value of asset

This will update the value of the custom field on each checkout and can be viewed by going to Reports -> Checkin/out Events

However if you plan on checking out multiple assets to a project in one go, we recommend using the Cart Add-On and adding a Custom Field to the cart (More -> Custom Fields -> Cart) to keep the data more manageable and easy to access.