We are new to laying tile on a kitchen floor. We will be putting a new subfloor of Hardie board over the original floor (we live in an older mobile home). We are leveling the floor but do not know how much mortor to buy foy=r a room that is 14x20. Any ideas?

I'd say 99% of the time tile would be a very bad idea in a mobil home.
The floors are so under built, they flex, with tile there has to be 0 deflection.
Cement board is not a subfloor it's an underlayment, and adds 0 strength to the floor. It's just there to give the thin set something to bond to.
What's there now on the floor. If it's partical board it's not going to work.

What would work and would still look like tile is a laminite flooring that has a tile pattern.
Very DIY and could be walked on as fast as you lay it, unlike tile.

IF, your existing subfloor is strong enough, then you can figure the amount of thinset needed by looking on each bag. Coverage will vary depending on what notch size trowel you use and size of tile will determine the notch size. Most floor tiles in the 8" and up range will use a 1/4" square notch trowel. Just take a look on the bag of thinset and it will give you an estimated SF coverage for each bag.