How many patience do you have?
One pot can cover quite an area if portioned in rather small pieces. And wen it takes of, it grows like weed and grows very dense.. I have a slightly smaller tank 60 x 30 and used 1 pot to cover 1/3 of it. (front carpet).. A year later i had to pull over 4 pots out to make space because it was killing eachother.
So one pot would have taken me a year to plant the whole tank with, with uprooting and good replanting strategicaly.
The closer you plant it togehter the sooner you come to the point you need to pull out again. And seeing it's propagation characteristics you will maybe pull out more then you like. Maintaining a nice healthy hairgrass carpet requires some work, more then only trimming the tops. From what i experienced, i'll do it differently th enext time, like uproot and replant sooner to not let it grow to dense into eachother.
Plants grow and aquariums are rather long term projects if not only for showcase display.. Imho it's better, easier and cheaper and healthier to work with that plant growth than work against it. If you plant to much to start with it might grow out of control sooner.
And if you want to cover the complete tank with hairgrass, still what you buy is emersed form and needs to go through transition.. Might be a lot or might be less, something you only can wait for and see, but probably some melt will kick in. For that reason also i would do it in stages and see how it performs in your parameters before putting in to much. In my tank i had some trouble to get it going, but did read journals where it took off from day one like a rocket.