My 18 yr experience is that one cassette will last two chains. Your mileage will vary but the two chain rule works for me. My mileage is
in the 3-4k miles for chains and hence 7-9k miles for cassettes in 8 or 9 spd. Only have about 10 k miles with 10 spds on two bikes
but one cassette started skipping in the mid range at ~8500 miles and was replaced last week (on new 3d chain for about 1500 miles
without problem til past 2-3 weeks.)