As many as possible, there is no exact answer to your question. I hold 2.4 million for staking for example.
No exact answer means no minimum coins to hold to start stacking, right?
We can start with 1 EMC?
Yes, you can start with just 1 EMC, just that is highly unlickely that you get stacking rewards because you are competing with larger holders, the more you have, the higher the probabilities that you get an increasing amount of EMC stakes.