Due to maximum friends that facebook allow/allows, I would like to give chance to those who want to watch my battles.
Which one should be used here?
Note that the verb agrees with "Facebook", not with "friends". For the purposes of verb agreement, companies and organisations (such as Facebook) may be treated as singular or plural (plural is less common in AmE, I believe). However, in your case "allows" is better in my opinion.
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Note that the verb agrees with "Facebook", not with "friends". For the purposes of verb agreement, companies and organisations (such as Facebook) may be treated as singular or plural (plural is less common in AmE, I believe). However, in your case "allows" is better in my opinion. "Facebook" needs a capital.
There are various other problems with the sentence. For a start, "Due to X, Y"