In the US, the plurals cacti and octopi are almost never heard. Cactuses and octopuses are sometimes heard for plurals, but they are awkward. In colloquial speech, the words cactus and octopus are used as plurals. For example:
That hill is covered with cactus. You've got a lot of cactus in your garden. There are a lot of octopus in this area. We've caught a lot of octopus