According to the FAA's Aerospace Forecast that was recently unveiled for fiscal years 2019 through 2039, the active general aviation fleet is expected to hold steady with 211,800 aircraft in 2039, compared with 212,885 aircraft in 2018. Fixed-wing turbine aircraft are expected to grow at a rate of 1.8% per year, single-engine piston aircraft decline at a rate of 1% per year, and rotorcraft grow at a rate of 1.7% per year.

General aviation hours flown are forecast to increase from 25.6 million in 2018 to 30.3 million in 2039, an average annual growth rate of 0.8% a year. In addition, fixed-wing turbine aircraft hours flown are expected to grow at a rate of 2.5% per year, single-engine piston aircraft hours flown decline at a rate of 1.1% per year, and rotorcraft hours flown grow at a rate of 2% per year.

For more: https://www.faa.gov/data_research/aviation/aerospace_forecasts/media/FY2019-39_FAA_Aerospace_Forecast.pdf