How much attack would I have?

These +/- boosts are called Stages, which are in-battle modifier for stats. Stages go from -6 to +6 and, from Generation III to now, have the following multiplier effect on stats:

| -6  | -5  | -4  | -3  | -2  | -1  | 0 |  1  |  2  |  3  |  4  |  5  |  6  |
| 2/8 | 2/7 | 2/6 | 2/5 | 1/2 | 2/3 | 1 | 3/2 |  2  | 5/2 |  3  | 7/2 |  4  |

However, these modifiers only apply to Attack, Defense, Special Attack, Special Defense, and Speed. In battle, your Pokemon also have Accuracy and Evasion stats, which follow different multipliers. As of Generation V, they are:

| -6  | -5  | -4  | -3  | -2  | -1  | 0 |  1  |  2  |  3  |  4  |  5  |  6  |
| 1/3 | 3/8 | 3/7 | 1/2 | 3/5 | 3/4 | 1 | 4/3 | 5/3 |  2  | 7/3 | 8/3 |  3  |

Thus for your example, your Mega Garchomp's Attack of 408 with a +2 boost would result in an Attack of 816.