Although OP explained it briefly I think still you are confused. Read below quote for easy explanation.
1: what's the maximum number of users I can add to my trust list?
There is no limit, you can add if you want all forum members.
2: that which the back black arrow is pointing at what does it mean?
This is the depth, you can select up to depth 4. I will explain it below
3: that which the red arrow is pointing at, what does it mean and what's the maximum number to write there?.
When you will add custom list it will be 2 by default. If you want to change you can do it. You can select from 1 to 4. What happend if you select 1 or 4? For your information you can't select any specific depth to specific user. Means if you change depth on your custom list then all your listed user will change. Now you have to decide what number you should select. Better depth 2 only.
Let's say you add me on your custom list. But you select depth;1 then you will see only my feedback. Whoever in my trust list their feedback will not visible to you.
Let's say you select me depth;4, now you will see my feedback and up to more 3 level who is my trust list. Like I add qwk, qwk add LoyceV , LoyceV add suchmoon. So you will see upto suchmoon feedback if you select depth 4. If you select depth 3 then you will see feedback upto LoyceV. If you select depth 2 then you will see feedback upto qwk.
Note: The feedback and rating will visible only to you. Not by all forum members. But if you are in DT1 Then all forum member will able see up to depth 2 feedback on current trust system.