I do not have a better answer than this one: Use M-x customize-variable RET flycheck-checker-error-threshold to change the threshold. Mine is currently set to 400.
If someone has a better solution, I gladly remove this answer.
Try to using command SPC SPC then search for FlyMake Mode and select it. Selecting it will toggle the mode on or off. Spacemacs makes finding commands simple by filtering down the list of commands as you type it in. For example, do SPC SPC then type fly. It will filter all commands that have "fly" in it's name. You might need to do this time to ...