There's no accurate number. It's better to fabricate soft working stamp.  And use working stamp to do nanoimprint.

There are examples of many thousands of imprints being done with a stamp.  Note, however, that it may be necessary to clean the stamp periodically.  Your stamp lifetime will be strongly affected by the defect level you can tolerate.  If your process can tolerate a higher defect level, your stamp life may be extended.