What happens when more miners start mining?

More miners is a good thing to spread the responsibility of securing the Bitcoin blockchain across more entities. If there are more competitors then each miner gets less rewards over the near term time. However, the difficulty adjustment system periodically adjusts for this to normalize things for the mid-term time. From the longest timeframe (for miners), their success depends on many things: their optimized investment in mining equipment (to include cooling, overclocking, advanced power management), electricity costs, contracts with electricity supplier, regulatory environment, external investment (whether stock or private), and operational efficiency. Many miners are part of mining pools which aggregate mining rewards and share them. This evens out the cash flow from Bitcoin mining.