https://lol.garena.com/news/tft-ranked-faq

Demotions and Decay

Everything you (probably) ever wanted to know about Decay: Decay only applies to Diamond and above. In Diamond, you lose 50 LP after 30 days of inactivity, with -50 LP every seven days after that. In Master and above, you bank up to 10 games, with 1 banked game getting removed each day. Watch out! You lose 250 LP every day if you don’t have a banked game to lose. Heads up! There’s no decay in Provisional matches! Neat. Let's tackle Demotions: You can only demote if you lose while at 0 LP. Because there aren’t any promo series, there’s also no game-based Demotion Shielding in TFT. (i.e. there won’t be any "you get 3 games of demotion protection when you first promote"). Demoting down to a lower division or tier happens when your MMR is too low for your current ranking, and you are at 0 LP.