Windows NT (3.1, and a number of later versions) only support 64 TLS
keys. Starting with Windows 2000, they added another page per thread for
TLS, giving an addition 1024 TLS slots, for a total of 1088 TLS slots.
FWIW, Win 9.x supported 80 TLS slots. See

http://www.nynaeve.net/?p=181
http://msdn.microsoft.com/en-us/library/ms686749.aspx
http://bugs.python.org/file11141/thread_nt.patch

TLS slots are typically considered a scarce resource, so that
programming language implementations typically don't allow applications
direct allocation of TLS slots. Instead, most runtimes I know of will
allocate a single TLS slot for themselves, which then is filled with an
array or a dictionary.