This number changes every day. What are the variables that affect that?
The minimum nomination stake is not a fixed number. The minimum stake of a nominator for an election round is the smallest nomination stake that is backing at least one validator that will be part of the next election round. Thus, it will change depending on the nomination bounds at a specific election round.
This PR (Staking: store last min-active-bond on-chain #12889) that has been recently merged, stores the minimum active stake for each round in the runtime storage, which makes it easier for e.g. wallet UIs to provide an estimate of the minimum nomination stake to earn rewards at a given time. If you look at the PR code, it will be clear how the minimum active stake is calculated.