There are a few ways that you can get new Scorestreaks in Call of Duty Mobile. The first way is by completing the game's various multiplayer challenges. These challenges will reward you with credits, which can be used to purchase new Scorestreaks from the in-game store.
Another way to get new Scorestreaks is by levelling up your character. Each time you level up, you'll earn one credit towards purchasing a new Scorestreak. You can also find credits scattered around the game's maps, so be sure to explore thoroughly if you're looking to stock up on some extra streaks.
Finally, if you're struggling to obtain any new streaks through these methods, you can always try your luck with the game's randomised supply drops. These contain a variety of items, includingScorestreaks, and are awarded to players at regular intervals simply for taking part in multiplayer matches.