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1. Create as many as you want, although my experience of partitions on a real Amiga weren't too good, caused a few crashes. But it's up to you.
2. No not really, just create folders for these interests if you want. The more partitions you have the greater the loss if these partitions fail.
3. The boot partition is necessary, this is where workbench is installed 'n' needs to boot from and load in. Without this you'll have to boot from a floppy disk to access the HDD. You can put everything you own into this partition, that's if you don't create any others.
4. I'll let the experts advise you on this one. My Amiga days are very rusty
