| Online, PS+ required |
In order to spawn a sidekick, you must first obtain enough money in the multiplayer match in order to afford it. Use your touchpad to spawn one in, and this will count as one of the ten you'll need. Gradually, this will fill in as you play early on in the multiplayer so don't focus too much; if you have any loose money that you can spare, drop one in every now and then so you can knock this out as your multiplayer experience continues.
Original post: It may say Multiplayer, but it does count the number of Sidekicks that you spawn during the Trials as well. With that being said, you can get the majority of these by trying to knock out the Trials by Fire trophy (by completing the Sidekick trials prior to playing online with others).
*I was made aware that, as of the start of 2020 or possibly earlier, the tutorial trials may still count for this trophy, so just keep trying.*