How many commercial jetliners are equipped with ADS–B?
The majority of commercial airliners are currently equipped. I don't have the latest statistics, but for Europe over 80% of the airliners were equipped with 1090 Extended Squitter ADS-B two years ago. The percentage in the USA was similar but a bit lower. 

That doesn't mean that all those aircraft are compliant with the European rules that mandate ADS-B for these aircraft from 2020 onwards. To be compliant, the aircraft need to have ADS-B version 2 (EUROCAE ED-102A / RTCA DO-260B compliant transmitters) and the installation in the aircraft has to meet certain criteria. At this moment (May 2016) I estimate the percentage of rule compliant airliners to be less than 20%. I haven't run statistics on my own dataset for a while, I may update this figure over the summer. 

In Europe, all aircraft heavier than 5700 kg maximum take-off mass or a maximum cruise speed exceeding 250 knots true airspeed are subject to the ADS-B rule.
Therefore private jets and some small propeller aircraft will need to equip with ADS-B as well. In addition many people voluntary equip their aircraft with ADS-B; it is not uncommon to see ADS-B equipped gliders in Europe. 

The situation in the US is a little different. The FAA mandate requires aircraft flying in certain airspaces to equip with ADS-B. In my view this makes more sense since it will prevent a mix of equipped and non-equipped flights in one airspace. 

In the USA, two types of ADS-B are allowed. The 1090 ES is required above 18 000 ft, below that altitude Universal Access Transceiver is also allowed. This causes all kinds of interoperability problems that can only be fixed at high cost.

The FAA mandates aircraft in the 48 contiguous states (and D.C.) that operate in class A, B, or C airspace or class E airspace above 10 000ft (class E airspace below 2500 AGL excluded) to be ADS-B OUT equipped by January 1st, 2020. The mandate itself can be found here and the associated Advisory Circular (AC90-114) can be found here. 

This means many more aircraft are subject to the rule. I have no recent information on what the implementation status is.