The composition of a fleet is determined by quite a lot of things, and pretty much all of them are circumstantial.
Level of threat
Type of threat
Number of fleets needed
Number and type of ships available
Strength of individual ships
Personnel available
Power of owning nation
Money devoted to fleets
Political climate (war, peace, various places in between)
If I have to have at least 3 fleets to complete 3 objectives at a time, and I've only got 10 ships, then there's a real good chance I'm going to have fleets consisting of 3 or 4 ships apiece. I'm usually a balancer, so I will try and even out the powers of the ships as best I can between those fleets, but not everyone does that.
One must also understand the difference between "fleet" and "task force". Most of the time, when some one says "fleet", they actually mean "task force". A Fleet is a large collection of ships, responsible for a large area or space. A fleet will have ALL the tools available that can be had: carriers, battleships, cruisers, PTMs, mine hunters, etc. Fleets are broken up into battle groups, which are broken up into task forces.
A battle group is usually like a miniature fleet, and is responsible for a smaller portion of the fleet's AoR (area of responsibility).
A Task Force is composed specifically for a particular mission. Deny the sea to the enemy here. Take that beach. Clear the sky. A Task force will go without one arm in favor of another, due to the anticipated threat. For instance, a task force tasked with making a landing will have a preponderance of amphibious ships, perhaps some extra battleships, and probably have no submarines and other anti-ship protections. A previous task force should have already cleansed the area of threats to friendly ships, so that the landing can take place.
A task force centered around a supercarrier is probably intended to control air space. It will have 1 or 2 carriers (maybe 3 or 4 or even more if the threat level is high enough) and SAM ships at its core. It will have picket SAM ships. It will fly air patrols. The fighters will probably be stealthed or not radiating, to make them harder to spot, and have powerful spotting aircraft to guide the fighters in to incoming threats.
In modern day, a task force that contains a carrier is going to have anti-sub vessels as well. You don't want to lose something as valuable as a carrier, so you have to treat such a task force as a battle group. Indeed, you don't really break up a carrier battle group.
A modern American CVBG tends to contain 1 carrier, 3 CGs or DDGs for air defense, 4 DDs or FFs for sub-hunting and picketing, and 1 or 2 SSNs for anti-sub and anti-surface. The carrier takes care of most threats with its aircraft. During the Reagan years, when we had ships aplenty, you could easily double the number of subordinate ships that protected the carrier, but nowadays, there are simply not enough ships to provide what I would consider to be adequate protection for the carrier. What keeps our carriers safe is the fact that so far our enemies haven't had the fleet power to muster any significant threat. If we were to go to war with France or the whole EU, the story would be different. Imagine the Soviets with a strong economy and the ability to stay ahead in the tech race, compared with our massive downsizing program.
Most other countries do not have world-wide fleets. We have like 7 fleets to cover the world with, most other nations have only their home waters to defend, and have thrown less money into the water as well, since we are there to protect them. Not many have carriers. Their fleets consist mostly of diesel subs and patrol boats and that's about it. But they will still compose task forces in the same way: based on the mission and the threat.
Composing a task force in Traveller is a bit more difficult, because the Imperium and its major rivals have enormous fleet budgets. There is little that isn't out of bounds. 100 battleships is a drop in the bucket. So it's going todepend on what you've got in mind.
Solomani Rim War? Frontier War? Interstellar War? Local brushfire war? In any case, determine the level of the threat posed, and the Imperium sends in a more powerful force. If the enemy has a preponderance of battleships, then maybe a matching preponderance of bigger battleships, or of carriers will be the rock that beats the scissors. Really, you're going to have to figure out what's going on, maybe playtest it a couple times, to figure out what's a good composition for a task force.