Where should I place the comma in large numbers?
It's 50 000 or 50,000, the same as international standard.

Reference: 出版物上数字用法的规定 (General rules for writing numerals in publications) 


  8    多位整数与小数: 8 Multidigit integers and decimals:
  
  8.1  阿拉伯数字书写的多位整数和小数的分节 8.1 Segmentations for multidigit integers and decimals written in Arabic numerals
  
  8.1.1  专业性科技出版物的分节法：从小数点起，向左和向右每三位数字一组，组间空四分之一个汉字(二分之一个阿拉伯数字)的位置。  8.1.1 Segmentation rules for professional scientific publications: from the decimal point, towards the left and the right, make every three digits a group; the width of the gap between two adjacent groups should be 1/4 of a Chinese character width (or 1/2 of an Arabic numeral width).

示例：2 748 456      3.141 592 65

  
  8.1.2  非专业性科技出版物如排版留四分空有困难，可仍采用传统的以千分撇“,”分节的办法。小数部分不分节。四位以内的整数也可以不分节。  8.1.2 For non-"professional scientific publications", if it is difficult to use "quarter space", the traditional segment comma "," is acceptable. Decimal part should not be segmented. Integers less than 4 digits can be either segmented or not segmented.

示例：2,748,456    3.14159265    8703 



Edit by Stan:

If following 8.1.1, then the 四分空(quarter space) should be used according to the standard. Unfortunately, the standard hasn't exactly defined which space should be used. The list of spaces can be found in wiki. And practically, when rendering NOT by a monospaced font, I suggest using


U+2005 Four-Per-Em Space, or
U+202F Narrow No-Break Space


for the "quarter space". Specially, the "Narrow No-Break Space" is for the intention of not breaking a digit number.

Rendering results with mixed font 宋体+Calibri via Microsoft Word are as follows:



and for the line-breaking case (text aligning setting: 两端对齐 Justify)


*Note that in the "Narrow No-Break Space" case, no matter how you edit the content, the number won't be break into two lines.

BTW, you can use the Alt and the numeric keypad to input the special space, like Alt+8197 or Alt+8239.