Question What is the max height of ram that can fit under the d 15 with 2 140 fans

The nh-D15s is a higher compatibility version of the D15.
https://noctua.at/en/products/cpu-cooler-retail/nh-d15s/specification

It differs functionally in a few respects.

a. because it is a single fan version, ram with tall heat spreaders are ok.
(If you have not yet bought ram, do not pay extra for fancy heat spreaders, they are mostly marketing.)

b. The NH-D15s is 160mm high vs. 165mm for the NH-D15.

c. The NH-D15s is offset a bit to clear graphics card backplates when mounted in the first pcie x16 slot.

From a performance point of view the D15 is a bit better, but not enough to be meaningful.