If you've lost your LLP's PAN card and forgotten the number, you can retrieve it by using your original NSDL application number. Check your emails from NSDL, as they often send digital e-PANs and correspondence with your details. Alternatively, you can apply for a new physical PAN card through the NSDL platform for a small fee.
25 May 2020
You also get Digital E-PAN from NSDL now a days. Moreover as already told you might also get the details of the same via Application Number, SMS, Email Correspondences. You also get a letter mentioning PAN Details from NSDL.