This is a great question and is an important factor in understanding bitcoin mining and value. All miners are affected by Mining difficulty which is "Bitcoin mining difficulty is a measure of how difficult it is to find a hash below a given target. The Bitcoin network has a global block difficulty where valid blocks must have a hash below this target."
As more miners are processing calculations, it generally increases the difficulty to mine a unit of bitcoin over time. However, in theory, the value of the respect bitcoin will go up.