I'd go for 16GB for 'future proofing' if you can afford it, but don't sell a kidney - 8GB will probably do the job.
I'm doing web development on a 2011MBP (upgraded to a SSD) with 8GB using NetBeans, and often use a Win7 virtual machine to test on IE and run some Windows graphics software for icon design. 8GB seems to cope perfectly well.
What are you using at the moment? If you're on a Mac, run Activity Monitor while you're doing what you do. Ignore the 'free memory' (OS X uses spare memory for file caching and stuff) - in Mavericks and later it has a handy green/orange/red 'memory pressure' readout. On earlier versions look at the
rate of 'Page ins' and 'Page outs' - if those are clocking up then the system is relying on disc storage to supplement RAM (see
https://support.apple.com/en-gb/HT201538) - that's when things grind to a halt on a hard-drive based system (not so bad with an SSD).
I'd say you probably don't
need more than 8GB of RAM unless you're dealing with bulky data (e.g. editing images at pro/print resolutions, working with big sound samples, HD/4K video editing). However, the fly in the ointment is the non-upgradability of rMBP RAM...