There is a simple value that translates between seconds and frames. If you want to increment a floating-point number evenly over time, you can use

number += Time.deltaTime;

Over one second it will increase by one.

However, if you want to increment an int exactly once a second, you can use a coroutine:

int incrementThis = 0;

IEnumerator NumberIncrementer(ref int number)
{
    while(true)
    {
        yield return new WaitForSeconds(1);
        number++;
    }
}

void Start()
{
    StartCoroutine(NumberIncrementer(ref incrementThis));
}

void Update()
{
    Debug.Log(incrementThis);
}

This way it will increment the number exactly once a frame.