Lido tries to distribute the stake evenly across all validators. Given a single delegation, the exact number of validators that will receive delegations and the amount that they will receive depends on the current distribution of stake. We take a sorted (ASC) list of validators, calculate the desired amount that each validator should have
target_stake = (total delegated + delegation_amount) / num_validators and begin adding stake up to the desired amount, starting from the validator with the least stake. The exact amount of a single delegation is calculated as
target_stake - validator_stake, and you'll have as many delegations as it takes to "drain" the delegation_amount.
You can check out the implementation of this algorithm here.