You should get 3-4 chains per cassette (without skip) and 3-4 cassettes per set of chainrings. 2,500 kms/1550 miles per chain is a rough guide, but use a chain checking tool. With Sunrace and Microshift cassettes you can get minor skip on chain #3 for a week or so before the chain settles down on the older cassette, while in my experience Shimano cassettes will often allow for four chains. SRAM I don't know about.

The lesson being, change your chain more frequently to maximize cassette and crankset lifespan. Some may change individual cogs on the cassette, but the interplay of new chain, new cogs and older, worn cogs can make for ugly shifting. It may seem like spending more money on consumables, but in the long run, it saves you money on new cassettes and chainrings/cranksets.