Learn Italian: Credo che l'affluenza sarà molto alta. - I believe that the turnout will be very high.

Verbs of thinking and believing do normally trigger the subjunctive, but the Italian subjunctive lacks a future tense. Because the high turnout is a future event, you must fall back on the future indicative (sarà). If the sentence were about the present ('I believe the turnout is high'), it would use the subjunctive: Credo che l'affluenza sia molto alta.

If 'to believe a fact' is often credere a, why is there no a before che?

When you believe a noun, you often use the preposition a (for example, credo a questa storia means 'I believe this story'). However, when you introduce a whole new clause with che (that), Italian syntax requires you to drop the preposition. You can never put a directly before a declarative che, so it is mandatorily deleted.

Is affluenza a false friend to the English word 'affluence'?

Yes! While they share a Latin root meaning 'flowing toward', they mean very different things today. In English, 'affluence' means wealth. In Italian, affluenza refers to a flow of people, making it the standard word for voter 'turnout', 'attendance', or an 'influx' of visitors. If you want to talk about wealth in Italian, use a word like ricchezza.