APR maximization mechanics are based on validator scores described above. Lido will renominate validators as short-term parameters changed, so that we are up to date with new scores each week.
According to our short-term and long-term simulations and also according to the rules of the Phragmén algorithm we nominate 3 validators from each ledger.
Nominating less than 3 validators leads to the risk of nominating a validator not elected to the active set. Being in an active set means that a validator has been elected to produce blocks at this era: https://wiki.polkadot.network/docs/learn-nominator
And not producing blocks means you will not earn any rewards for that era: https://wiki.polkadot.network/docs/learn-staking-faq#nominated-validators-not-in-the-active-set
Having more than three validators leads to lower average APR in the available validator list left.
The next thing that we consider in choosing nominations amount on each validator is era points randomness. According to our simulation result, if we renominate validators every 7 days, then fewer nominated validators on each ledger leads to higher APR.