Even if the ledger can index a large number of accounts, it has a finite capacity. To avoid saturation and maintain good performance, some accounts are trimmed if the ledger capacity is reaching a threshold. This mechanism prevents dust (i.e. accounts with balance too small to be spent) from accumulating in the ledger.
While inserting a transaction, if the number of indexed accounts exceeds a given threshold, a fixed number of accounts with the lowest balance will be trimmed from the ledger. The balance of each of these trimmed accounts will also be deleted (a burn transaction block will be added to the chain for each of these accounts).
For the SNS ledger, the threshold is currently set a 28 million accounts for the ledger, plus 100,000 for the trim quantity. Therefore, if adding a transaction lead to index more than 28.1 M accounts, the 100,000 accounts with the lowest balance will be trimmed and their balance burned.