Are there enough Ripples for everyone? There is a finite amount of Ripple tokens (XRPs). If I recall correctly the amount is 100 billion XRPs.

Opening an account requires 200 XRPs.

So, are there only enough for 1/2 billion accounts, ever? Can the numbers be tweaked later on? Could XRPs be divided below 1 XRP?

Was this problem discussed anywhere? (E.g. on the Ripple forums)
XRP can be divided into millionths. The reserve levels can be changed by consensus. By the time this becomes an actual issue, the system will be managed by consensus and there's no way to predict what stakeholders will decide to do. The rough expectation is that they'll try to keep the cost of opening an account consistent with the actual cost imposed on the network by having an additional account.

Also, just to point it out again, those 200 XRP aren't lost. You can use them to fund transactions. You just can't transfer or exchange them. The reserve is designed to discourage ledger spamming.