A problem by Muhammad Imran Gondal

A fort has 7 gates. On each gate certain number of guards are there. One day enemy attacks on the fort and tries to enter the fort from gate 1. The captain on the gate 1 requests all the captains of the other 6 six gates to send as many guards at gate 1, as are already present at gate 1. So they all fight against the enemy, and defeat them. After that enemy decides to attack at gate 2. The captain of the gate 2 requests all the captains of the other 6 six gates to send as many guards at gate 2, as are already present at gate 2. Enemy is again defeated at gate 2. Then the enemy decides to attack at gate 3. The captain of the gate 3 also makes the same demand for the other 6 gates. The process continues, until the enemy is defeated at gate 7. When the enemy is defeated at gate 7, and went back, the number of guards standing at the each gate is the same. Find that number