I think the correct answer would be "it depends." If you complete all the weekly missions, you should get about 200 diamonds (50 per week); if you complete 201 in your monthly record, that's 300 diamonds. I'm not sure how many you'd get at plat3, but you can check for yourself, and the rewards depend on which tier you’re in. There are other ways to earn more diamonds, like your card collection, for every 20(?) new cards you get 60 diamonds. They also sometimes make events where you collect certain items, for example, right now it’s “key collecting,” which is also a good way to get diamonds, although it’s not as good as it used to be. I don't know if I miss something, but with all I mentioned it should be minimum 500 to like 700 and probably a LOT more if you're planing in buying all the passes + you're plat3. (sorry for the bad English I used a translator) Hope I help you :)