Buffs, stats (health/mana/ego), resistances, regeneration and similar benefits or maluses are based on a 10 level system. If a skill or ability lists two numbers, it most likely means it is on the 10 level system (ex: 1/5, 2/8, 4/4, etc.).

The first number is the actual level. The second number is the maximum level that particular ability counts towards. For example, 1/4 fire resistance means it is a level 1 fire resistance and will give +1 fire resistance up to level 4. 

If you have a 2/2, 1/1 and 1/2 fire resistance, it will only count as a level 2 fire resistance because none of the resistances count over level 2.

If you have a 1/3, 1/1 and 1/2 fire resistance, it will count as a level 3 fire resistance: 1/1 gives the first level, 1/2 gives the second level (as it can go to a max of 2), and the 1/3 takes you to 3 (as it can go to a max of 3).

Do not worry what order you apply your resistances, it will always be calculated to your maximum benefit. 

Generally, the maximum level is 10, though certain artifacts can take you up to level 13.