Why do wallets of Bitcoin exchanges have tens or hundreds of thousands of addresses?
Is that number fix ?


It isn't. New addresses are generated by exchanges all the time.


  What those addresses represent exactly?


They are simply multiple addresses controlled by them. They can represent addresses the users deposit coins too (often one per currency, so BTC and USDT may have separate addresses), or intermediary addresses used for consolidation, or hot wallets, or cold wallets, or temporary addresses used as change addresses.


  Why do we find so many addresses but only around 20 of them are rich?


In general, exchanges tend to have thousands of user addresses, into which users send crypto. Exchanges will then move all that crypto to a handful of hot and cold wallet addresses, which they will then use to process use withdrawals. The majority of addresses will be empty most of the time, unless you happen to check one between the user depositing coins and the exchange sweeping it to a consolidation wallet.