ODing on Ketamine is quite difficult. LD50 for mice i.V. is 77mg/kg. So for an 80kg mouse that would be more than 6 grams of Ketamine given i.V. typically the bioavailability by nasal or oral consumption is lower and Musk most likely has a high tolerance. So we are talking probably 50grams or more of Ketamine for a lethal dosage for Musk.
I am not sure if Musk could actually snort enough Ketamine for a lethal dosage without it just falling back out of his nostrils or if he could swallow one without his stomach turning before it is adsorbed.
Obviously all of these numbers are speculative and taking drugs is harmful, no matter the amount. Also people often die not by ODing on Ketamine, but by killing themselves in accidents as they loose control over their muscles.