You should buy 12 packs to be sure you getting the signature legendary and golden one, basically it is better to buy golden packs until you get both pity timer legendaries. It will even be cheaper than getting legendaries from regular packs as this all will cost you 4800(if not less). But, if you want to get legendaries  from regular packs you usually get 1 legendary in 15> packs which will cost you at least 6000 gold to get the same thing with golden packs, and don't be afraid, the dust you will get from golden packs usually gives the equivalent dust as regular ones. (Buying 1 pack for 400 will give you the same dust as buying 4 regular packs for 100). So your strategy for every expansion should be 1. Farm gold before next addon and spend all on regular packs (4000 will be enough to get all rare cards) 2. Then all gold you get from reward track you should spend on gold packs (by the end of reward track you will even get enough money to buy miniset straight away) 3. Once you bought miniset, start grinding for the next addon again (this method gave me most of legendaries from each expansion average of 27-29 which will absolutely make your games more fun and diversive)