Every asset a Stellar account holds raises the XLM it must keep locked

How trustlines drive up your locked XLM

Holding a token on @StellarOrg's network comes with a cost that is paid before any tokens arrive. Trustlines are an explicit opt-in: to hold a specific asset, an account must establish a trustline with the issuing account using the change_trust operation. A trustline must be established for an account to receive any asset except lumens ($XLM).

Stellar accounts must maintain a minimum balance to exist, calculated using the base reserve. An account must always hold at least two base reserves, currently 1 XLM, and every subentry after that requires an additional base reserve of 0.5 XLM. Subentries include trustlines (for both traditional assets and pool shares), offers, signers, and data entries. So each new asset a user holds, each open trade offer, each additional signer and each data entry all add to the minimum balance in the same way.

An account cannot have more than 1,000 subentries. That ceiling applies across all subentry types combined.

Lowering the reserve and sponsorship options

The minimum balance is not fixed permanently. The minimum balance is held in reserve, and closing an entry frees up the associated base reserve. If you zero out a non-lumen balance and close the associated trustline, the 0.5 XLM base reserve that secured that trustline is returned to your available balance.

As a practical illustration, an account with three trustlines must maintain at least 2.5 XLM: (2 + 3) x 0.5. Removing those trustlines via the change_trust operation brings the minimum back down to the 1 XLM baseline.

There is also a delegation route. Stellar's sponsored reserves feature allows another account to cover a user's reserve obligations, shifting the locked $XLM to the sponsor's balance rather than the user's. Sponsored reserves affect the minimum account balance and are reflected by the number of sponsoring entries and sponsored entries in the calculation.

The trustline mechanism helps mitigate spam and unwanted tokens on the network, and by requiring a minimum reserve it deters the creation of excessive or spammy tokens, preserving network resources.

Sources:
Stellar Docs: Understanding Lumens and Minimum Balances
Stellar Docs: Understanding Accounts and Subentries