$12K to $20K is relatively simple and moderate for folks around here. The typical point in which you would need to provide IRS 4506 form for a CLI would be somewhere in the neighborhood of $35K. If you're willing to support your request with the IRS doc you can go as high as they deem reasonable in relation to assets/debt/rent/income/etc.
$50K is a relatively easy thing to accomplish if you provide the doc or slowly work your way there over a few years.
More than $50K doesn't show up around here that often unless they're bored enough to come report it. If $50K is too high then they'll counter back after reviewing your IRS form. There's really no downside for shooting for the moon w/ AMEX as long as you have what you say you have. Now if they spot something like requesting $50K on a 30K salary and you said you make 50K then there might be more supporting documentation needed like bank statements and employer verification resulting in a reduction or closure if they can't get what they need.
Per a couple of people around here allegedly you can go up to $1M on personal cards even though I thought it would be significantly lower than that for 99.999% of us. The good news is you can go back for more in 6 months and roll the dice as you see fit 