STEP 1: How Much

Find the memory size supported by your computer . Run Windows Shell and type

wmic memphysical get maxcapacity

Copy the result and divide by 1024 one time and by 1024 second time. You have the result in GB

STEP 2: What RAM should you use ?

Run Crucial Scanner or do a test on User Benchmark

If the Scanner detects your RAM Memory it tolds you what is compatible
If not, take the specifications and search on Amazon the results of RAM Test.

EXAMPLE
I run a test on a Huawei Computer
Crucial Scanner Not Working
Result wmic 134217728 = 64 Gb
Click on RAM Page and in the right see Alternatives, in occurence i choose Corsair Sodimm DDR4 3200