Click the dropdown on the right side of the value and select Multiply Float.
https://i.gyazo.com/a44d618c8ae24dfe455e3336fd8a690a.png
This turns it into 2 values, where you can enter the spawn rate into one, and turn the other into your variable.
https://i.gyazo.com/3ffbac3c0142674794afdd48da30df71.png