The Tax field is added automatically, and its name depends on the region set on your device.
To set the tax percentage, tap "Tax" at the bottom of the invoice. From there, you can also choose whether the tax is included or excluded.
You can control which items are taxable — tap an item and toggle tax on or off for it.
There's no dedicated field for a tax number, but you can add it to the "Notes" field when creating an invoice.