Copied from help: (note you need to type "MaxHatch" and match case of each letter)
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Note By default, AutoCAD won't create a hatch pattern that consists of over 10,000 segments. The limit is set by the MaxHatch setting in the registry. To reset the limit to 50,000, for example, enter (setenv "MaxHatch" "50000") at the Command prompt. The limit can be reset to any value between 100 and 10,000,000.