Can I mix an 8GB and 16GB RAM stick in a ThinkPad T480 for 24GB total?

Yes, mixing an 8GB and 16GB stick for 24GB total is fine on the T480. The first 8GB of the 16GB stick runs in dual-channel with the existing 8GB stick, and the remaining 8GB runs single-channel. For everyday work, browsing, coding, or light gaming, you won’t notice the difference versus a matched pair, and having more RAM is more useful for juggling tabs, virtual machines, or Docker containers. A 2x8GB kit gives perfect dual-channel but tops out at 16GB, which offers less headroom for real-world use.