Yes. Both Dell specifications and Crucial's site indicate you may use 128 GB RAM at either 2666 or 3200 MHz. They do not indicate any restriction on 3200 MHz RAM to a lower total amount, so you can use 128 GB. You could buy two of these matched pair 64 GB kits for a total of 128 GB at 3200 MHz. Re. another question from your first post: no, you cannot use 4000 MHz RAM at its designed speed. That RAM might work, but it would be downclocked (its speed reduced) to 3200 MHz or lower. Whenever touching components or working inside a computer, wear a grounded wrist strap, also called anti-static wrist strap, ESD wrist strap, or ground bracelet. It's a cheap and sensible precaution. Rest the laptop on an anti-static mat or at least a reasonable alternative such as corrugated cardboard.
Will the DELL Precision 7540 with an i7-9750H processor support 128GB RAM at 3200MHz? Currently, I have 2x16GB 3200MHz, and it's working fine. Can the same hardware support even higher DDR4 memory speeds, such as 4000MHz?
Max MHz DDR4 on DELL Precision 7540