There are 54 skates. To unlock the "Skates collector level 3" achievement you have to collect at least 49 skates but In the current game version the developer has accidentally bugged the level 9 (Darkest Night) and it's getting skipped. You can't play it, even though on level screen it displays the level 9 as "SKATED" and that level contains 8 skates so in total you can only collect 46 skates which are not enough for the achievement.

You have to download the initial game version where the level 9 was playable. I suggest you do that after you've earned all the other achievements as the current version contains new content and new achievements.

1. Open the Steam console using the following link: steam://nav/console

2. Type: download_depot 444520 444521 4170473032468192544 and when the download finished the following message will be displayed on the console: Depot download complete : "C:\Program Files (x86)\Steam\steamapps\content\app_444520\depot_444521" (79 files, manifest 4170473032468192544).

3. Go to the game's installation folder C:\Program Files (x86)\Steam\steamapps\common\Leave Me Alone and delete the folder: LMA_Data and the files: LMA.exe & steam_api.dll.

4. Go to the folder where the initial game version is downloaded: C:\Program Files (x86)\Steam\steamapps\content\app_444520\depot_444521, copy the folder: LMA_Data and the files: LMA.exe & steam_api.dll and then paste them on the game's installation folder C:\Program Files (x86)\Steam\steamapps\common\Leave Me Alone.

You can now play the initial game version in order to collect all skates and unlock the "Skates collector level 3" achievement.

Note: If you're having trouble finding the skates check the following guide: https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=696046638