It's an indication that
no one can solo. Guild Wars is a team game. All missions and exploration areas are balanced for teams ranging in size from 2 to 8 members. A Necromancer or any other profession entering a mission designed for characters around their level alone has very little chance of success. That's why the game has NPC henchmen available that you can join with to form a group or to pad out an understaffed group.
Here's how minions work. First, to summon a minon you need to have a corpse. Then, you can summon one of three types of minions depending on what minion skill you have slotted. There's a ranged minion, a strong melee minion, and a weak melee minion (that spawns either more minions or at a cheaper cost than other minion spells). Then, there are many other skills which can buff, heal, or otherwise affect your minions. The level of your minon is tied to your Necromancer's attribute
Death Magic. It depends on the skill you're using and the minion you're summoning but your attribute will range from 0 to 16 (with a hat and a superior rune, item mods can push you higher, too, but I can't ever recall seeing a +1 to Summon whatever) and you can take that as a rough approximation of most minion's levels. This means they'll fall well short of the max level of 20 and since a character's level also influences just how much damage they'll deal - basically higher level characters will hit lower level characters harder while lower level characters will hit higher level characters softer - minions are not amazingly effective sources of damage. They're also uncontrollable. They attack whatever it s that you decide to attack or the nearest target. There's no way to tell them to disengage or to target different characters or any other sort of advanced tactics, they're a horde of brainless killers and they'll swarm all over a target until it dies.
What minions have going for them is numbers. By having minions you have more characters on your side than the other side (especially if the corpses you're exploiting belong to the other side) and as minions clog the battlefield they can trap opposing players or screen your more valuable teammates. They're not going to take out a player 1v1 but neither are they entirely ignorable. They're more than a slight nuissance because they do *some* damage and they can be dangerous in large packs but they're not much more than just simply annoying to a team of lv20 characters.
However, that doesn't really matter. A Necromancer is not just a character who'll pump out minions as quickly as possible. It's a much more diverse and complex profession which can be paired up with five other equally diverse and complex professions. You can play a Necromancer/Ranger that uses curses and hexes to debuff an opponent before finishing them off with a bow. Or a Necromancer/Elementalist that slings around DD spells. Or a Necromancer/Monk that uses a Necromancers ability to shift energy around with the Monks ability to shift health around to support the rest of their team. Necromancer/Mesmers can counter both melee and spell-casters. Necromancer/Warriors can use minions as nothing more than walking energy batteries with Soul Reaping. And each of those combinations can be reversed - you can play a Ranger/Necromancer as easily as a Necromancer/Ranger - as well as play completely differently from the narrow and limited ideas I've just suggested.
Look at Blackie's
Necromancer guide if you're interested in finding out more (It's not comprehensive, but it's a start). Then you can browse around our
Build Directory to get a feel for how some people play their necs.