Minecraft wiki entry for Exploring_caves, associated text: Lighting - Caves can be and will be very dark, so be sure to bring along any source of illumination. Rather than bringing many stacks of torches, bring wood and coal to craft them as needed. Stacks of sticks and coal would take twice as much inventory space if crafted into torches.
The deepslate layer is clean of any coal ore, so you'll have less reliable sources of light. You may have to consider working with the dimmer light levels of sources like glow lichen, redstone torches, and redstone ore. Producing charcoal is another thing to keep in mind.