World vs. World in Guild Wars 2 is not a game of numbers as it may seem at first. A single player can impact the WvW if they are careful. You can contribute to the war effort by running supply lines and smaller objectives that add up to give you the edge your world needs.
The biggest thing that can effect battles are supply lines. Each keep must be supplied in order for the keep walls to hold under heavy assault and for counter siege weaponry to be built. Without the supply, the keep will eventually fall.
To that end, destroying/defending the roaming supply dolyaks can help and taking the supply camps can cripple the opposing army. Likewise, supply is required to maintain a siege (to build siege weaponry), so keeping your own supplies up during an assault.
Sentries stand and guard random locations throughout the map. You can contribute to the effort by either attacking or defending a sentry. To capture a sentry point, defeat the sentry and then stand in the capture point until the progress bar indicates that it has been captured.
To defend a sentry, keep your sentry from perishing and hold the capture point. Capturing them provides points for your world and is something a smaller task force can easily accomplish.
You can solo a sentry, but it requires you to be careful as sentries have significant dueling potential. It becomes much easier 2v1.
Mercenary Camps are important for a number of reasons. They translate directly into more troops; boosting the effectiveness of even a very small force. They continually send out groups of attackers/defenders to nearby supply camps and towers; either to assist you in capturing or holding that location.
They offer a field merchant at their home location that allows you to sell your junk and even more importantly buy siege weapons.
To gain their allegiance, there are two events that must be completed. The first is the “gain the loyalty” event, in which you’ll need to kill enemies, resurrect allies, and more, which can be easily soloed. The second is the “decrease their morale” event (when another world already has their loyalty), which is a bit harder to complete solo and will need some reinforcements.
Resource Camps are moderately easy to take over because their only defenses are a few guards and have no walls or gates to protect it.
You can probably take over a resource camp with a small group of 5-10 players (keep in mind though it takes even fewer players to defend it effectively as the NPC guards are no pushovers).
Every map has six Resource Camps and award 5 points for each camp held every 5 minutes. They are one of the most important objectives in WvW. Without these Resource Camps, your team can not receive Supplies to build sieges, rebuild, or upgrade and capturing them will deny the enemy of these things.
All of these objectives might seem like minor things in a battle with hundreds of combatants but most of the time it will be the army that thinks tactically that takes the upper hand in WvW.