You can buy up to 30 normal mine shafts in each mine, and each normal mine shaft can be leveled up to 800. The warehouse and the elevator can each be leveled up to 2400 when you have normal mine shafts unlocked.

Additionally, after you have reached Prestige 5 in any mine on the seven continents and you have unlocked the 30th mine shaft there, you can then unlock 5 extra core mine shafts. These can be levelled up to level 800 too, and they also increase the maximum level of your warehouse and elevator to level 2800!