To get the value of the textfield try (c#)

int number=int.Parse(textfield.text);

Restricting a textfield to numbers is difficult. I would try this (but it certainly isn't very good code)

try
{
    int number=int.Parse(textfield.text);    
}
catch(Exception e) 
{
    textfield.text="0"
}

This code checks if it can parse the text to a number. If the convertion fails, the textfield didn't contain a number and is set to 0. Please do not comment about this, I know this is very bad style. I also know the sentence "Don't use exceptions for flowcontrol"

To disable parts of your game I would add public variables to the script that checks the textfield. This should work (c#)

using UnityEngine;
using System.Collections;
public class textdisable : MonoBehaviour
{
    public GameObject object1;
    public GameObject object2;
    public GUIText textfield;
    int number;
    // Use this for initialization
    void Start ()
    {
    }
    // Update is called once per frame
    void Update ()
    {
         if(Input.GetButtonDown("Enter")){
        try
        {
            number=int.Parse(textfield.text);   
        }
        catch(Exception e)
        {
            textfield.text="0";
        }
        if(number==1)object1.active=false;
        if(number==11)object1.active=true;
        if(number==2)object2.active=false;
        if(number==22)object2.active=true;
        }
    }
}

In the inspector you can assign objects to the variables object1 and object2 of your script. If the textfield contains specific numbers the GameObjects object1 or object2 are enabled/disabled. Lets say object1 contains the script that spawns the enemies in your scene. When the user types 1 in the textfield and presses "Enter" the object is deactivated and no more enemies are spawned.