"Standard" (most common) Hardie is 12' x 8 1/4", leaving a 7 inch reveal.
For 600 SF, you'll actually need to order about 700 SF of siding assuming you're doing normal **** joints. Always have to account for wastage with fiber cement, usually around 15%. Roughly about 96 pcs, as noted above.
Not sure where you're located, but a small batch like that is gonna run you full cost with no bulk discount. That's about $6.50/piece around here. So just the siding, about $625 before tax.
Don't forget to add cost for trim boards, plywood & housewrap (if needed), metal "plates" behind the joints, and Vulkem caulk ($5.00/tube). Plus you'll have to paint it.
Hardie isn't the cheapest material, but it is good stuff. I don't recommend it for DIYers.