How many targeted keyword anchor links do you think you can add in a month to a site before it throws up a red flag with google? I am referring to each key term. 10, 25, 50 ?
I think we would all love to have absolute numbers for something like this. From my experience, Google seems to be looking at trends over time. Google likes stability, so an overall steady, predictable growth in backlinks seem to do well whereas sudden variations from the historic growth trend will raise alarm (though backlink spikes will often will boost a site up the SERPs before they get knocked back). Jury is still out, but steady growth, at whatever number per day/month/whatever, seems to be the key.
I would guess that a gradual build up of backlinks would look more natural than you getting like 500 backlinks in one day. Work on building them every single day and you should be fine.