Hello @StelaClaudia Hearty Welcome to our friendly King Community
The Gifts -- Lives which we receive from our friends will be expired 7 days after they are received! So, if the number is decreasing at your envelope icon, it means, some of them have expired and hence being reduced.
If you keep sending lives to your friends, they will be sending them back and you will have many Lives stored in your envelope
Hope this made sense and clarifies! Please post back if you need further assistance. If you tap on "Type your Comment" box located below, you can reply back.
Thank you for posting in the Community! Have a great rest of the day/evening!