Can you trade in more than one device to get credit towards a new one?

It appears by reading the Trade-up program, it is a single device.

Get instant credit in store.

Take your iPhone or eligible smartphone to an Apple Store and get immediate credit toward the purchase of a new device.1

Trade-in values may vary based on the condition and model of your smartphone trade-in. Monthly payment may vary depending on condition and type of device you trade in and carrier financing plan chosen. Offer only available on presentation of a valid, government-issued photo ID (local law may require saving this information). You must be at least 18 years old. Value of your current device may be applied toward purchase of a new Apple device. Offer may not be available in all stores and not all devices are eligible for credit. To see if your device is eligible, please visit Apple’s approved partner for the trade in and recycling of eligible devices. Some stores may have additional requirements. You will be required to agree to the full terms and conditions available at the Apple Retail Store. Online and in-store values may vary. Apple reserves the right to refuse or limit the quantity of any device for any reason.

As was provided above, you can not trade multiple devices toward a single device from Apple. I know this from personal experience as well. Now, that is with Apple directly. Other sellers may have differing rules for trade-ins and you should check with them directly.

Renew and Recycling - Apple

iPhone Trade-up - Apple

Ask whomever you are planning to trade if they will do this.

If I have a iPad mini 2 can I trade it in to get like and iPhone SE

The policy is one device only.