How many electrons must a magnesium gain or lose to achieve electron configuration?

The way the question is Worded makes little. A magnesium atom has an electron configuration whether it is negatively charged, neutral, or positively charged.

Electron configuration is not a fixed definition that only allows for one certain number of electrons in order for it to "be achieved," simply the fact that it has any electrons at all means it must have an electron configuration.

In its neutral state its electron configuration is: 1s22s22p63s2 (or abbreviated: [Ne]3s2).