enemy spawn and in game time counter

To spawn a GameObject, you can either create them on the spot using Instantiate or create a group of them at the start of the game, store them in a pool then use/spawn them whenever you want.

For the timer, just create a float variable and set it to the value of each spawning interval at start. Inside the Update method, minus the value with Time.deltaTime. Once the timer reaches 0 (or lower), call the code for spawning the enemy and set back the timer to its original value.

Not quite sure what you mean by "a in game timer counter", but I'm guessing something like how long have the player play the game? If yes, just use the same logic as the spawning timer except that, instead of deducting the value until it reaches 0, you start at 0 and adds the value of Time.deltaTime inside update.