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As Glen aready noted, the limit is nine for data stacking (StackWise). 3750-X power stacking (Stack Power), between stack members, is limited to 4.
Also note, large 3750 stacks can sometimes be very sluggish doing some CLI commands.
And also note, although StackWise Plus supports "spacial reuse" for unicast traffic, large stacks might congest the ring sooner.
Lastly, for very large stacks, a chassis might be a better and/or less expensive solution.
