Laws of Cricket 1947 Code (Fourth Edition) - Law 1
A match is played between two sides of eleven players each, unless otherwise agreed. Each side shall play under a captain who before the toss for innings shall nominate his players who may not thereafter be changed without the consent of the opposing captain.
Notes on Law 1
1. If a captain is not available at any time, a deputy must act for him to deal promptly with any points arising from this and other Laws. 2. No match in which more than eleven players a side take part can be regarded as first-class, and in any case no side should field with more than eleven players.
Reproduction of the Laws of Cricket is by kind permission of Marylebone Cricket Club