You can give upto 4 SAN when creating CSR from iDRAC. There is only one field on "iDRAC Settings > Services > Web Server > SSL Certificate" page tp specify SAN. On this field you can give to upto 4 SAN by comma separated. If you are creating CSR and certificate outside iDRAC then you need to upload the private key to iDRAC first using "racadm sslkeyupload" command then upload signed certificate.
I am aware that in iDRAC 9 world (4.10.10.10(Build 32)), one can create a SSL certificate request for an iDRAC through iDRAC Settings > Services > Web Server > SSL Certificate > Generate CSR but can only put in 1 Subject Alternative Names (SANs). I would like more than 1. The documentation on creating CRS through RACADM is very poor: it appears as though using racadm with sslcsrgen you cannot define anything, let alone multiple SANs. I have also used DigiCert Utility to create a certificate with multiple SANs  and export it with the private key as a pfx. However, importing this pfx really breaks the iDRAC as the certificate is so untrusted, I cannot use any web browser to connect to it. I had to use racadm to bin off the pfx certificate. Is there a better way?
Multiple SANs for SSL certificate