The flasher manufacturer just want you to provide fuse protection for your flasher in the case that a wire gets grounded out or the load is higher than the flasher is rated for.
Grote states a min of 1 Amp fuse, a max of 20 Amp ... as stated on the flasher.
Passenger Cars/Light Trucks LED Compatible EL12, 20mA~25A; Electro-Mechanical Flasher; 1 to 12 Lamps; 400 Hours; 12 Volts, 300 Watt
This flasher is rated for 300 Watts.
Using the formula:
Amps = Watts/Volts
Amps = 300/12
Amps = 25
So, if this flasher is capable of flashing 12 bulbs, (25/12) the 2 amps per incandesant bulb makes sense.
LED's will draw significantly less, so IMO I'd start with a 5A fuse and go up from there if it blows.