It is not clear to me what you want to do:
1) Do you just want recommendations for the ampere rating for the fuse at 12V? If so, that has been give in posts 3&4.
2) The schematic you show, however, is a boost converter. Do you want to increase the power and speed? If you boost the voltage to 18 volts, the motor will consume considerably more current -- perhaps beyond the motor's safe operating power. The circuit you show will need to be modified; although, the modifications needed may not be that great. In all likelihood, you will probably need a different inductor for the higher current. The mosfet shown is rated at 50A and 60V, so it may be able to handle that power.
This datasheet for the LM3478 used as a boost converter is offered as an example and describes the calculations needed:
https://www.electro-tech-online.com/custompdfs/2012/06/lm3478.pdf
A more general discussion is here:
https://www.electro-tech-online.com/custompdfs/2012/06/snva035a.pdf
Designing a boost converter to go from 12V to 18V at that higher, as yet unknown current will take some time. TI ( National) does offer several online design tools to help. Is that a project you want to launch?
John