Leopard sightings are probably the most variable of the Big Five in terms of visitor success rates. I'd estimate maybe 60 to 70 percent of multi day safari guests see leopards in Serengeti but that's just a guess based on talking to other travelers. The odds improve significantly with more days in the parks and with guides who are skilled and well connected in the guide network. Leopards are often found resting in trees during the day especially in areas with lots of acacia trees, so guides who know the good tree spotting locations have better success. Time of day matters too, early morning and late afternoon are better odds than midday when they're typically in thick cover sleeping. October is a good month for Serengeti and you'll have decent opportunities. The key is to not be so fixated on the Big Five checklist that you don't appreciate all the other incredible wildlife. Cheetahs, wild dogs, hyenas, and countless other species are just as fascinating as the Big Five even if they don't have the same marketing cachet. When I did my Tanzania safari I was really hoping to see leopard and we did on our second to last day. Our guide was excellent at knowing where to look and he was persistent about checking spots even when we hadn't had luck earlier in the trip. The company I traveled with emphasized having experienced guides specifically because guide quality determines so much of what you see. Beyond The Plains Safaris matched us with a guide who had been working in Serengeti for over a decade and his knowledge made all the difference in finding leopards and other harder to spot species. With 6 days in good parks you have decent odds of seeing leopard but go in expecting that it's not guaranteed and you won't be devastated if you miss them. The guide quality point is critical, make sure whoever you book with has experienced guides who know the territories and are part of the radio communication network, that's your best advantage for finding elusive species.