Example in pure go of read a ENV VAR and leaving default value if not exists (In that case already convert from string to integer)

main.go

import (
  "os"
  "strconv"
)

func main() {
	tc := 1
	if temp := os.Getenv("THREADS"); temp != "" {
		number, err := strconv.Atoi(temp)
		if err != nil {
			panic (err)
		}
		tc = number
	}	
}