As others have said, new accounts have exaggerated gains for a time (18-25 games, including placements). After than, they settle down to +/- 20-30 per match.
Note that it is also fairly common for genuinely new accounts to fall down from initial placement, as all new accounts’ first game is at 2350 or so, which can be optimistic. Don’t worry if this happens to you.
More information is at How Competitive Skill Rating Works (Season 13), but that is probably more than you need right now.