EMA only has one number: it's period.
If you want the EMA to follow prices closely, you use a small number. This is called "fast."
If you want the EMA to smooth over price bumps, you use a larger number. This is called "slow."
You may even want two EMA's on the same chart—one fast and one slow. Then you watch the crossovers.
But there is no single right period to use.
This Investopedia article says, "The most commonly used EMAs by forex traders are the 5, 10, 12, 20, 26, 50, 100, and 200."
So you may as well start there.
Period 25 shift 8, period 15 shift 5, period 9 shift 3, all calculated on median price.
And don't focus on the crossovers, look for when price starts making clear one-directional zig-zags on one side of the EMAs and the EMAs start spreading.