TOBUFSIZE limits the total number of recipients that can be delivered at once. It sets the size of the buffer that will hold the list of recipients, where that default size varies based on your operating system. If you need to increase that limit you can experiment by cautiously increasing TOBUFSIZE.

To change the size of TOBUFSIZE, edit sendmail/conf.h and rebuild sendmail. There is no debugging switch that will show the size of TOBUFSIZE.