That is the important observation, and question. The reason that we have so many different visa classifications is primarily from a misguided government attempt to react to market demands for labor. Quotas have been modified annually, and the bribery and corruption at USCIS has barely been exposed. The question about how many we can afford is easy. If one views immigrants as a human asset, we can afford approximately 200,000 per quarter as long as the economy is growing at 3-4%. If one views immigrants as a human liability, fire up your buses and line up the porta-potties. Unemployment will stabilize at 7-9% if you deport 200,000 (working age) per quarter. In 13 and 3/4 years (the year 2020) you will have accomplished your deportation of 11 million, and underemployed roughly 20 million Americans. Feel free to tweak my numbers and come up with a better scenario.
All the while my taxes will go up to support this illegal immigration and to bolster someone else's business