A box contains three small balls of the same size but different colors. It is known that the number "1" is marked on the red ball, the number "2" is marked on the yellow ball, and the number "3" is marked on the white ball. Xiao Ming Draw 10 such balls from it. After calculation, the sum of the numbers on these 10 small balls is 21. According to this, do you know how many red balls there are at most in the box? Tell me your reasons.
一只盒中盛有三种大小相同，但颜色不同的小球，现已知红球上标着数字"1"，黄球上标着数字"2"，白球上标着数字"3"，小明从中摸出10个这样的小球．经计算，这10个小球上的数字之和为21．据此，你知道盒中至多有多少个红球吗？说说你的理由．
<result>4</result>