As said it’s possible to limit number of jobs in compilation (simultanous tasks) in /etc/portage/make.conf
MAKEOPTS="--jobs n"
where n is number of jobs (threads). 50% less jobs means 50% less required RAM. However limiting so much does not makes sense. Good CPU needs a lot of memory, so if you are not ok with it then use other CPUs as otherwise the memory would be a bottleneck in this or any other situation …
For sure you cannot limit memory usage as there are pre-emerge checks for that. Of course there are no requirements like that for most of the packages, but that’s because most projects follows UNIX way of writing software i.e. small apps around one feature, so in practice you only need to be careful when compilling webview, browser, office or other “blob” of (definitely useful) features.
