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Gowkanapalli JanardhanreddyGowkanapalli Janardhanreddy 

How to disable new button when i get 20 records in table

pconpcon
To do this, you would have to override the standard layout with a custom visualforce page.  Then inside the controller for that selectively show / hide the button based on the number of records.  Another (more fragile) option would be to do a simple visualforce page that include that default layout and some javascript.  Then have the javascript disable the button based on the record count returned to it from the controller.