Learn Italian: Il mercato nero è più lucroso di quanto non si ammetta. - The black market is more lucrative than one admits.

This is the pleonastic non (an extra, non-negative non). After a comparative of inequality like più... di or meno... di, Italian often adds non before a conjugated verb. It emphasizes the contrast being made, but doesn't actually negate the verb!

Why do we use di quanto here instead of just che to mean 'than'?

You use di quanto (or di quello che) when you are comparing a quality to a whole clause or action, which translates roughly as 'than how much' or 'than what'. Since we are comparing the market's profitability to the entire action of people admitting it, we need di quanto.

Why is ammettere conjugated in the subjunctive mood (ammetta) instead of the indicative (ammette)?

Clauses introduced by di quanto or di quello che in a comparison require the subjunctive. This is because the second part of the comparison usually deals with an assumption, expectation, or abstract idea (what one might admit) rather than a concrete fact.

What is the function of si in si ammetta?

This is the impersonal si. It is used to make general statements where the subject isn't a specific person, translating to 'one admits' or 'people admit' in English.