If all humans were vegetarians what effect would that have on the number of people that the earth could support?

The elimination of poultry, livestock, and fish as food resources would mean that humans would have to find substitutes for things like wool, leather, and Gelatin. While these may be seen as valuable products, they are actually by-products of animal slaughter and would not be cost-effective to produce from animals if they were not being sold for meat.

The amount of food energy used to feed livestock animals (grains) could feed 10 billion people every year.

Some other possible effects of ending livestock production:

• Substantial tracts of land would become available to wildlife. Over time, the world's farmed animals would be displaced by countless species of wild animals. Predator control programs would end, and wild predators would keep the population of wild herbivores in check.
• Decreased deforestation and increased reforestation. According to the Food and Agriculture Organization of The United Nations (FAO), livestock production is one of the major causes of the world's most pressing environmental problems, including global warming, land degradation, air and water pollution, and loss of biodiversity. You can grow 11.6 times the amount of protein on an acre of land planted in beans and rice versus an acre of land devoted to grazing cattle.
• Less irrigation would be needed to sustain the smaller number of farm crops. One pound of beef requires 2500 gallons of water to produce, as well as up to 16 pounds of grain and soybeans. Less fresh water would be lost to evaporation.
• More genetically-engineered foods may be developed, more resistant to various conditions, having a greater yield.
• Decreased production of certain products (gelatin for example might be produced less depending on the alternate source, agar, which comes from algae.)
• Better nourished world population - Depending on location, greater crop yields available for human consumption and not fed to animals could cause lower infant mortality and lower birth rates. The less land required for animal food production, the more there is for forests, wildlife, homes, and buildings.

This is only a partial answer- there would be MANY MANY more benefits if everyone went vegetarian.

One thing that would happen is there would be less animal cruelty. Animals everyday wouldn't be beaten to death, if it weren't for someone who wanted a "nice" meal on the table. Some people would become less violent (especially the ones who kill animals for money), and people everyday wouldn't have to face the horrible truth of animal cruelty. Since there are no longer jobs for killing helpless animals, we would be able to focus on more important areas like economics, health, ect.

I have considered that thought many times over, and the world would be a better place- if everyone went vegetarian.
http://michaelbluejay.com/veg/environment.html One pound of beef takes 5,000 gallons of water... if that answer is so off there... it makes me wonder exactly how wrong the rest of it is... This website contains a LOT of information about how everyone becoming a vegetarian could cut down on greenhouse gases and end hunger in 3rd world countries...
No more factory farming. The world would not become overrun with animals because they would no longer be raised for food. Most vegetarians are caring, compassionate people. So I guess there would be more peace, less violence, and better attitudes toward animals.