In Front Mission's OCU path (the only one of the two campaigns I have played so far, and the original one), you start with one character, though you quickly recruit four more characters. However, as the game goes on, you continue to recruit more and more characters. As I recruited more characters according to the plot, I noticed that the number of units I could bring into battle also increased to match at first. This even tipped me off that I had missed an early optional character. However, once I had recruited the 11th character, I noticed that the cap on the number of units I could bring into battle remained at 10. So, I logically concluded that 10 was going to be the maximum number of characters I can bring into battle.
Now then, I should bring up a few details on how the system of the game works. The only way characters can build up experience points at any rate at all is to bring them into story missions. While there is an arena, it drops paltry amounts of xp. Each mission can give a character 2 or three levels. While it is possible to catch up characters who fall somewhat behind, it can be tricky if the enemy has a big advantage. Furthermore, outfitting you combat characters with the latest mechs and weapons can be very expensive. Whenever new parts come in to the shops, I usually end up broke. So, there are clear motivating factors to not bring more characters than is necessary into a mission. It is much more difficult than it is worth to rotate characters in and out of your line-up. Front Mission isn't that different from later Tactical RPGs in this regard.
However, after fighting several missions at the ten cap, which I had assumed to be what I would be dealing with for the rest of the game, and recruiting several more characters, I suddenly ran across a mission that let me bring 11 units. So suddenly, I was short one unit for a mission. I didn't want to go to the trouble of buying equipment for a character several levels behind, so I just ran the mission with my prepared ten. However, the very next mission suddenly dropped the limit down to 8. Now, I couldn't even bring all of my prepared characters. And this came right after I had just outfitted all of them with new mechs. And since new machines became available just after than mission, I squandered thousands of dollars worth of money on equipment I never got the chance to bring into battle. And the next mission I have to go through will only let me bring 5 characters. If the mission after than lets me bring 10 again, half my party is going to be seriously under-leveled.
All of this is a serious problem with game design. Players tend to plan ahead based on patterns that emerge as a game unfolds. If every dungeon has a boss at the end of it, players will plan their resources accordingly. Constantly messing with those expectations randomly and unpredictably, particularly in a way that has a serious impact on game balance such as party size, simply frustrates the player. Future Front Mission games corrected this problem. Front Mission 4 allowed me to field all of my units every mission. No guess-work or weeding out of characters required. I think even the UCS path in Front Mission (which was added to the game in its Playstation port) was adjusted to have a fewer number of characters that are more predictably around too.
Side notes on Front Mission so far:
Between Front Mission and Final Fantasy 2, I am pretty sure that I really don't like "abilities level up as you use them" mechanics. It can be too frustrating building abilities up the way you like. I'm afraid that I have gimped my main character already.
Whenever new mech parts become available in Front Mission, I quickly discover that only two or three mechs from a batch of five to seven are actually viable. Sometimes new, more expensive parts are worse than older ones. Inevitably, it is always the dorky looking ones that are the useful ones too. Its frustrating. I think the problem was still around in Front Mission 4 too. Someone needs to go back in time and teach Square about balance.
On the other hand, having a powerful villain figure with a kick-ass mech showing up every once in a while to taunt or threaten the heroes always works in mecha stories. It certainly worked for Char, and Driscoll is doing a fine job in that role with his Type-11DS.