Pinyin capitalization when sentence starts with a number
This may be hard to answer definitively. (It might be documented in the ISO 7098:2015, but it's behind a paywall.)

In any case, it would be natural to transliterate a sentence beginning:


  23万人...


as


  25 wàn rén...


We certainly know sentences start with capital letters:


  6.3.1 句子开头的字母大写。例如：
  - Chūntiān láile.（春天来了。）
  - Wǒ ài wǒ de jiāxiāng.（我爱我的家乡。）
  
  中华人民共和国国家标准，汉语拼音正词法基本规则，2012。
  (Basic Rules of the Chinese Phonetic Alphabet Orthography, 2012.  Source.)


In the given example, the sentence starts with 2 (not 万).

It also seems reasonable to believe that pinyin follows English capitalization, like it does for other forms of capitalization (start of a sentence, people's names, etc.).



Some related observations...

It is possible to use Arabic numerals in pinyin, the above document contains this example:


  635 fēnjī（635分机）
  (extension 635)


along with examples where the pinyin corresponds to what is pronounced:


  èr fēn zhī yī（二分之一）
  èr líng líng bā nián（二〇〇八年）


I didn't find a definitive example where a numeral in Chinese was transliterated to its pronunciation in pinyin.

And there's also examples of place names where Chinese characters are converted to numbers in pinyin:


  二马路 (2 Mǎlù)
  三环路 (3 Huánlù)
  中国地名汉语拼音字母拼写规则（汉语地名部分），1984。
  (China Place Name Hanyu Pinyin Letter Spelling Rules (Hanyu Place Name Part), 1984.  Source.)