Alright, I'll answer more specifically, generally, because IMO the total number of quality links changes from niche to niche, and in some it's natural to have more links than others as noted previously...
One of the biggest things I try to think about and I think is often not discussed enough are some of the points made in those old patent applications, which are specifically vague, but IMO give some insight to the thought process behind the methodology... Patterns are IMO one of the keys moving forward, so I would say the max number of links you can add is the total number of quality links / the time you want to build links before you hope pure natural growth replaces your need to build links.
One of the applications talked about two sites, each with 10 links and one being linked a year ago at the time of it's creation and one being linked now and the difference in the timing of the links added being a 'signal' of what's going on with each site right now. Link churn is another one, which talks about the rate of links added compared to the rate of links deleted being another signal.
Where everything gets a bit complicated and difficult to say a specific pattern of 'correct' for is when you throw QDF (query determines freshness) into the mix, because 'freshness' cascades much like PageRank, so a fresh link from a fresh page increases the freshness of 'older' content. Many people seem to think 'freshness' is always better and 'freshness' means 'new' WRT content, but that's not the case.
One example would be oiling a hinge... There are only a certain number of ways to do it, and a page with 'old' content presenting all the different possible ways may be an authority on the subject and receive links because of the 'inclusiveness' it has, which keeps the 'old' content fresh, so even someone publishing a new page about an 'old' subject does not necessarily make the content 'fresh' for the results generated.
Also, sometimes 'stale' is preferred in the results, so the aged, old, static links on pages that never change may be more of a benefit by keeping the page 'staler' than a brand new page with brand new links from new pages that are always changing...
If you go get a bunch of 'super fresh' links from Digg, Reddit, Tweeting, you will generate 'super fresh' which could be exactly what you need, but, in order to maintain the level of freshness over time, manage your link churn, and present 'still a hot topic' you have to maintain the inbound link pattern, otherwise the pattern is peaks and valleys until you build links again.
I had one page's traffic double from Diggs, but they were unsustained and the traffic only lasted about a week... The rankings of the page for certain terms increase in a similar pattern. The links weren't something I did, but had a huge effect on traffic right away, but it was a back-burner site, so as soon as the links were 'not the top thing' the rankings dropped back and the traffic went away.
Anyway, I would say figure out what you need, 'fresh', 'super fresh', 'relatively stale and stable', then figure out how many links you could build if you needed to, divide by the amount of time you want (or need) to build links before you think 'natural' growth will kickin and keep you at the level you need to be at to continue to rank and go from there.
It's a really complicated situational question for me to try and answer too much more specifically, because IMO to do it right, you have to do it in a sustainable, niche specific, search type specific way...
If it's a 'hot topic' type site, then press releases monthly and continuing quarterly while tweeting daily might be the answer, but if it's a content and information site to compete with something like WebmasterWorld, then solid, consistent, quality, 'trusted', aging, 'long-term-growth' oriented links are probably the best for you to build along with some tweets or something. If it's a 'how to oil a hinge' type site, then IMO tweeting daily would look pretty spammy and might do you more harm than good? IDK, don't have one of those, but it's an idea of how something might work well for one site and totally backfire on another.
I think there's a different 'method of madness' I would try to apply in different situations and a different type of link I would try to 'control' or 'grow' or 'focus on' personally while letting some of the other things 'take care of themselves' a bit more, but whatever I did I would try to make it sustainable, because if your natural link growth isn't enough to cover the lack of links you personally supply as soon as you stop building the pattern of links indicates 'less exciting' and that could be reflected in the rankings, maybe not today, but in 6 months or a year.
An example would be if you build 10 links a month yourself and pick up 10 links a month naturally... Your growth rate is 20 links per month until you stop building, then it's only 10, so the pattern is 'less interesting' over time, but if the natural link growth picks up and you go from 10 natural links a month to 100, then stopping your 10 link a month building campaign has much less impact.
I think that's about the best answer I can give, because I think it really depends on the specifics of the site, niche, situation, and even time you have to spend on it consistently.
Hope I make a bit of sense and give you an idea or two from my non-answer to your specific number question...
