There are 101 stickers in Just Dance 2021. Stickers are unlocked in two ways:
* playing one of the JD 2021 songs twice
* randomly getting one as a reward in the Gift Machine
Speaking to point #1, when you play a song for the 2nd time you'll unlock a sticker as a reward. There are 41 basic songs in JD2021, but you also get stickers for the alternate/extreme versions of songs. As such there are 53 songs that you have to play twice to get all stickers. Note that it doesn't matter how well you play, so long as you record a score (can even be 1-star) twice on each of the 53 songs you'll get the sticker.
You can tell if you've unlocked the sticker for the song by looking at the song in the Songs menu. When previewing there are three "Song Rewards" icons, the first (a happy face) if you've ever played the song, the 2nd (a semicircle) if you've played the song twice (this is the icon that will be coloured if you've unlocked the sticker), and the 3rd a happy face with dots around it for megastarring the song. If the middle icon (the semicircle) is coloured and not grey then you've unlocked the sticker for that song.
Ok so that's 53 stickers, that leaves another 101 - 53 = 48 stickers to go.
Unfortunately these are random in the gift machine. For me, by the time I unlocked my 101st sticker I had 6 items left in the gift machine, so you'll likely unlock this right around the time you unlock the "Gotta Get 'Em All" achievement for unlocking all gift machine items.
Note that you can track your progress at any time by going to the profile menu, and looking at the sticker album. There's a count on the sticker album option on this menu that tells you how many of the 101 you've already unlocked.