How many days of tribulation did Jesus tell the church in Smyrna to expect?

Answer: Ten

The devil would throw some into prison, and they would suffer tribulation for ten days. "Be faithful even unto death."

“Do not fear what you are about to suffer. Behold, the devil is about to throw some of you into prison to test you, and you will suffer tribulation for ten days. Be faithful even unto death, and I will give you the crown of life.”— Revelation 2:10 (BSB)