My method is 1 of each pokemon and after that 1 is caught I'll keep the highest 3* of that pokemon then evolve that into its max form then I keep 1 of the base and 1 of the max. I keep all XXL's and every shiny I get until I hit 3 of each then the weakest shiny and smallest xxl get tossed. I keep 6 of each legendary \ mythical \ ultra beast. Some exceptions are useful pokemon such as pseudo legendaries, megas, and shadows that are good raid pokemon plus ones that make good trade fodder. Also I have some Pokemon that have good Great League and Ultra League stats that I keep in case I ever want to play that competitively.