1) It's only 14k people, for activity between 2013-15. There are likely to be further disclosures for later years, depending on what the IRS finds for the years they are already looking at. They may also request information for more customers in later years if the number of customers that meet their criteria for suspicious activity goes up. Previously the IRS requested info on pretty much anyone, but now after court review the summons has been restricted to the customers with potentially suspicious transactions (see below).

2) According to the court order, the summons requests info for customers “with at least the equivalent of $20,000 in any one transaction type (buy, sell, send, or receive) in any one year during the 2013-2015 period.” Buy and sell transactions of BTC meaning a person put dollars in their account to convert to BTC or took BTC out of their BTC wallet for dollars in their Coinbase account. Whether they then withdrew it from Coinbase to their bank account is irrelevant for tax purposes because they either realized a gain or a loss upon the sale so they had to report it as income if there was a gain.

So Coinbase will run a query aggregating the transactions by year, account, customer, and type, then produce all the records for any customer with any item over $20k, except those who had only buy transactions because the summons does not cover people who only bought and held.

For send/receive transactions, the $20k will almost certainly be the FMV at the time of the transaction. That is easy enough to calculate based on transaction time, transaction BTC amount, and price of BTC at that time. The FMV should be recorded as a matter of course in Coinbase's ledgers, though I am not an insider so I do not know for certain. I would be very, very surprised if their accountants did not insist on recording that information on some ledger for book purposes anyway. The buy, sell, and withdrawal transaction amounts very definitely are recorded on their ledgers and that amount will simply be the nominal amount at the time because they are denominated in dollars anyway.

If you did ten sales worth $10k total, no that wouldn't be covered. Twenty sales worth $20k would be, even if you had a corresponding $20k of buys that netted out to zero gains. In that case, Coinbase will give your info to the IRS who will then take no action because there was no income to report and thus no failure to report it.

Which is to say, the IRS can request someone's info, but that does not necessarily mean the IRS will take any action or that the person has done anything wrong. The IRS has simply requested info for people whose activities can be categorized as higher risk for tax evasion. They will then compare the account information with filed tax returns to determine if the person has failed to report any income. Some people will have reported their BTC gains. Others will have no realized gains. Most people will probably fall into those categories.

A very few will have very complicated records with lots of BTC sent and received which may require an audit to sort out. Some of those will be legitimate BTC users, others may be using their BTC transactions to disguise complicated financial transactions from the IRS. We'll have to see, though I am interested to see how it all shakes out.