tmpfs will use by default your ram. you do not need to specify it.
https://en.wikipedia.org/wiki/Tmpfs

to -j <number>:
there is no general rule, it always depends on the "other" softwares running on the host and also depends on the speed of the hardware (not only CPU, but i/o speed).
One single task will not be able to use the full power of the system, because there are i/o wait time and probably other issues. Running several tasks will allow the cpu to work continuously, without waiting for i/o or other things. One task is always the kernel, you need to take into account all the daemons and background processes (not to speak about X and the terminals...)
giving lower value (to -j) will cause idle times, giving (a bit) higher values (as the optimal) will not slow down the build process.

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the other thing to bear in mind is sometimes more than make -j1 can ruin the compile (rare but does happen) and sometimes setting j greater than number of processors can influence the build (cache full) see gcc documentation about this. i use -j12 (8 cores) with gcc on a extremely profiledbootstrap.

If you want to squeeze every last drop out you can profile every package build (i usually run 40 or 50 automated makes till i find the one i want :-D

Good Luck!!!

Emma