In this task, you're given a context, further information available on a particular linked term from the statement, and an answer term. Your job is to generate a question that can use the information provided to obtain the given answer. You should use the information on both context and link information to create the question. Note that the answer to the question should be exactly the given answer, and if the answer is none, the answer to the question shouldn't be obtainable from the context or linked information.

Context: form the nucleus of the Boston Red Stockings, a charter member of the National Association of Professional Base Ball Players (NAPBBP) Link Information: Boston – Boston Red Stockings (1871–1875)
- Chicago – Chicago White Stockings (1871, 1874–1875)
- Forest City – Cleveland Forest Citys (1871–1872)
- Kekionga – Fort Wayne Kekiongas (1871)
- Mutual – New York Mutuals (1871–1875)
- Athletic – Philadelphia Athletics (1871–1875)
- Forest City – Rockford Forest Citys (1871) (A second league club with the same name as the Cleveland entry)
- Troy – Troy Haymakers (1871–1872)
- Olympic – Washington Olympics (1871–1872; 1875)
- Atlantic – Brooklyn Atlantics (1872–1875)
- Eckford – Brooklyn Eckfords (1872)
- Lord Baltimore – Baltimore Canaries (1872–1874)
- Mansfield – Middletown Mansfields (1872)
- National – Washington Nationals (1872; 1875) Washington Blue Legs (1873)
- Maryland – Baltimore Marylands (1873) (played at Madison Avenue Grounds)
- Philadelphia – Philadelphia White Stockings (1873–1875) (also sometimes called "Pearls" or "Phillies")
- Resolute – Elizabeth Resolutes (1873)
- Hartford – Hartford Dark Blues (1874–1875)
- Centennial – Philadelphia Centennials (1875)
- Elm City – New Haven Elm Citys (1875)
- St. Louis – St. Louis Brown Stockings (1875)
- St. Louis Reds – St. Louis Red Stockings (1875)
- Western – Keokuk Westerns (1875)

 Answer: 23
How many charter members were there in the National Association of Professional Base Ball Players?