Arnold Is Numero Uno

the font used in Arnold is Numero Uno T-shirt

Although I could not identify the exact font, style you want is often called "Varsity," "Collegiate" or "university."

Three verrrrry close matches (the only major difference is the central peak of the letter "M" is flush with the base line, instead of floating in the middle):

http://www.dafont.com/allstar4.font?text ...
http://www.dafont.com/pop-warner.font?te ...
http://www.dafont.com/bou-college.font?t ...