Firstly you must check which is the maximum RAM that your processor supports (usually 32 or 64 GB). You can use HWiNFO for this (look at Central Processors>your processor>Memory Ranges>Maximum Physical Address Size.
The only sure way is to not use them at all. There are ways to fight telemetry (e.g. O&O shutup Windows 10 - a helpful program) but you can never be sure that your system isn't "leaking".
Avira is a relatively small in size and light in performance impact antivirus software. The free version doesn't have web protection but there are some browser add-ons (I use Avira Browser Safety with Firefox).