*** Please note, the numbers in this guide apply to a "Normal" difficulty campaign -- for harder difficulties, the amounts shown are (approximately) halved.
... this achievement is attained by reaching 80 scientists in your Research Lab during the course of one campaign. Similarly, the fastest way to acquire scientists is to launch satellites over countries that will provide you with scientists every month for maintaining those satellites.
The continent that is scientist-heavy is Europe, as you will receive an extra 8 scientists per month for having satellites over all four European council nations.
In addition to Europe's contribution, you will also receive scientists every month for maintaining satellites over other countries, with the following reward structure:
+4 scientists/month for satellites over all three of the North American nations.
+2 scientists/month for satellites over both South American nations.
+2 scientists/month for satellites over all three of the African nations.
Asia offers no scientists, as this continent's reward structure is engineer-heavy, aiding greatly toward the above-linked "One Gun at a Time" achievement.
As you can see, once you have a full network of satellites up over at least all twelve scientist-contributing nations, it won't take many months for you to reach 80 scientists (at +16 scientists per month, in addition to whatever bonus scientists you choose to play for).
One may also acquire extra scientists by receiving rewards for certain types of missions and council requests; all such rewards are one-time rewards, however, whereas the satellite coverage bonuses repeat every month.