2
Even so i think that one will be plenty enough, i would buy two anyways.
Just imagine you are running out of it, for whatever reason, when you are 2 inches short of the end!
I think, compared to the total amount of work, effort, skill, sweat, thoughts, time and all the other things, the price for a second sika cartdridge is to be neglectable.
The answer to your question, therefore, is two!!