Reusing Old Torches
Like any other block, torches can be removed from their position. Left click while pointing the targeting reticle at the torch you want to remove; this will cause the torch to disappear and a small two-dimensional representation of the torch to be thrown on the ground close by. Run over this representation to pick up the torch; you can then place this torch wherever you want.
Crafting New Torches
You can create torches by placing coal and sticks in a crafting window. Position the two crafting materials in vertically adjacent squares, with the coal above the sticks. You will produce four torches for every one coal and stick you use. You can craft torches anywhere, provided you have the raw materials in your inventory.
Alternative Light Sources
If you have access to buckets and a source of lava, you can dig holes in the ground and place lava in them, providing light to your immediate surroundings. Glowstone blocks will also provide light when placed. Another alternative is to place flammable blocks, such as wood, and set them on fire using a flint and steel.
Considerations
Removing some or all torches from areas you already have visited to place them elsewhere can be an effective way of dealing with a lack of torches. On the other hand, if you are in a complex cave system, removing torches may cause you to become confused as to which areas you have already visited and, ultimately, become lost. Additionally, if you are playing on a difficulty other than peaceful, monsters may spawn in the areas you remove torches from and attack you when you least expect it, causing you to die.
Crafting additional torches is an easy way to solve a lack of them. On the other hand, sticks require wood to craft; wood can only be found from trees, and trees only grow naturally above ground. If you run out of torches deep underground and have no wood or sticks with you, you will have to retrace your steps to the surface to collect the materials to make new torches.
Using lava for illumination can be dangerous. If you accidentally pour the lava outside of the hole, it may touch you and cause you to catch on fire and die; you may also die due to accidentally walking into a lava pool you created for illumination. Placing Glowstone blocks for illumination is safe; on the other hand, Glowstone is hard to obtain, requiring access to the Nether dimension, and should only be used as a light source as a last resort. Any block that is on fire will eventually burn out, making flammable blocks inefficient and at best a temporary light source. The only block that burns forever is Netherrack; however, much like Glowstone, it requires access to the Nether dimension to obtain.