From now on how many egld should I stake to have 5 egld’s in average ?

It depends how much you had so far.

At 1% per month staking interest you need 500 egld

If you stake it now, then you'll have 7 days which it will get counted. If you don't get it in today, then you'll have 6 more days of snapshot. To get a little more than average of 5 you can stake: 10.75 EGLD (If stake soon) 12.55 EGLD (If stake 24 hours from now)