Go ahead and add the 16GB stick.

Yes, mixing an 8GB and 16GB stick for 24GB total is fine on the T480.

The ThinkPad T480 has two DDR4 SODIMM slots and supports flex mode. That means the first 8GB of the 16GB stick will run in dual-channel with the existing 8GB stick, and the remaining 8GB of the 16GB stick runs single-channel. For everyday work, browsing, coding, or even light gaming, you won’t notice the difference versus a matched pair. You’ll just notice having more RAM – which is way more useful for juggling tabs, virtual machines, or Docker containers.

A 2x8GB kit gives you perfect dual-channel but tops out at 16GB. That’s less headroom, and the tiny performance bump from full dual-channel rarely matters unless you’re doing bandwidth-hungry tasks like video editing or high-FPS gaming. In most real-world use, 24GB beats 16GB every time.

Future You can always upgrade to 2x16 later if you outgrow 24GB. For now, spend the money on the bigger stick.