Actually Earth's current population is over 6.8 billion. It's projected to reach 7 billion within the next year or two. The growth seems to be slowing, but it's still likely we'll pass 10 billion before the end of the century.
True - of course, at 10 billion, I fear for the quality of life on Earth.
However, by federation standards, today's Earth is overcrowded.
In 'Articles of the federation', it is established that there are 155 federation member worlds - the 'core' of the federation, so to speak.
Among them, Deneva with "only" 100 million inhabitants, is established as a federation member world, represented in the federation council. And it's a human world, alongside Earth and Mars - apparently, once a 'colony' is sufficiently developed, it gains 'member world' status;
The horta - millions of them would be quite a feat, as far as population growth is concerned.
And there are probably other examples of member worlds with population far below 6 billion (what about Bajor?).
I counted EACH member world as numbering 4 BILLIONS.
The colonies - we never saw or heard of a colony world which counted more than a few tens of millions.
I counted 2 BILLIONS colonists for each member world (including Deneva, horta, Bajor, the spider-looking species that joined the federation at the end of 'Articles of the federation', etc).
And even with these severely infalted numbers, I couldn't even reach 1 TRILLION!
Add to this the fact that 'Destiny' established the population of BOTH Alpha and Beta quadrants as numbering a few trillions (7000+ densely inhabited planets and space structures) and the number you get is FAR below the UNSUPPORTED 9.85 TRILLIONS!