How does one play this trill in this Mozart Sonata? Does it mean we play 3 notes (Eb, F, Eb) before the last two notes of the second bar? Or how many trills does one have to play?
Yes, you'll want to play E♭–F–E♭ before the final two sixteenth notes. My personal suggestion is more along the lines of:
But some purists will insist on the rhythm of the D–E♭ being precisely as written, and therefore:
This is where your own interpretation comes into play. I recommend listening to as many recordings of this opening as you can find; you'll find several interpretations of how others have approached this trill, and they can help inform your own interpretation.
User 11684 very helpfully pointed out that the above interpretations are actually not consistent with performance practice of the Classical period. Their answer deserves your attention! Oddly, my four recordings of this sonata all include the top interpretation above.
It seems that the historically accurate interpretations would begin on the upper note, suggesting either:
Both other answers overlook a crucial point.
In Mozart (and contemporaries and earlier composers) trills should generally start on the upper note, which is the case here. However, many professional pianists do not know this and start on the lower note anyway. This is, historically, simply wrong. There are plenty of sources on this, for example CPE Bach’s Versuch über die wahre Art das Clavier zu spielen talks about this, I believe.
Yes, it’s fast, but it should be “F Eb F Eb D Eb” (with extra “F Eb” added if you have time, but I guess not). The good news is that the sixteenth notes probably are just there to indicate a nachschlag, so you can play them later and shorter than written.
Well, if you can fit two trills there, it's certainly better. At least it's what some performers do, despite the fast tempo. In that case, you certainly do not care about the timing inside the beat and play it basically as one long ornamentation of the quarter Eb note.
I hope my ears don't betray me and here it's really played as Eb--Eb-F-Eb-F-Eb-D-Eb:
The ABRSM edition (pub 1982) - an Urtext edition, as much as is possible with Mozart - has these notes, from Stanley Sadie:
Editorial realizations of ornaments are shown in small notes above the text at the first occurrence of the ornament concerned in each movement. These realizations are based on the leading sources contemporary with Mozart, such as CPE Bach's Versuch über die wahre Art das Clavier zu spielen (1753-62), Leopold Mozart's Versuch einer gründlichen Violinschule (1756) and Daniel Gottlob Türk's Clavierschule (1789)
... in a trill a player should feel free to play more notes, or fewer, as seems right. No ornament that feels awkward to the player, or sounds clumsy, is being satisfactorily realized. A player who wants to vary the realization of ornaments more extensively ... should note that except in very rare circumstances a trill should begin on the upper note in music of this period
The editorial realization given has demisemiquaver (32nd note) F E♭ F E♭ then semiquaver (16th note) D E♭.
Every performance I can find on YouTube is in line with this. The awkwardness involved with a repeated E♭ at this tempo would (IMO) sound terrible.
I must respectfully disagree with almost all the recent comments above. CPE Bach says the following: All embellishments... must not corrupt the purity of the voice leading (Eulenburg edition p 95 point 17 and his examples figure 81). In this case the melodic line is a very obvious ascent up the C minor arpeggio (C to Eb to G) and therefore the F would be a most illogical/ unwelcome note to accentuate or focus on. In music there are always competing rules and we must weigh up (through our ear) which rule offers the most musical solution.