in theory, your tax basis is what you paid originally plus all subsequent items reported on your k-1 of income less deductions less distributions except for the final one. then in theory the liquidating distribution the selling price should equal your tax basis.
this will not work if you acquired your share by inheritance or gift. then it's the tax basis for the shares you were gifted or inherited +/- the same items as above but the liquidating distribution could result in a gain or loss.
the IRS has actually come out with form 7203 (for 2021) for use in computing tax basis but it will also work if you prepare the form for each year you had an ownership interest.
https://www.irs.gov/pub/irs-pdf/f7203.pdf
it is also possible to have a different gain or loss for alternative minimum tax purposes.
