Is there a maximum amount of playable Characters available?

If you use the source code version of UFE you can create your own character selection screen.

Simple way, but you'll have portraits at default size:
In the Inspector for the CharacterSelectScreen object, the Characters Grid, Grid Columns and Grid Rows are what controls the layout and size. 

Default grid is 4x2, so if you changed it to 3x2, you should see the default four characters take up two rows (Kyle gets pushed to 2nd row). 

If you want to go larger than default 8 character grid, you'll also need to change the Characters Grid width and height to compensate.  By default, it's only large enough to view a 4x2 grid, but if you changed the height to 335 or so, you'll get another row.  Of course, you'll probably want to adjust the x and y positions as well, as it's aligned from its top left corner, so won't be centred on screen.

Slightly harder method (involves coding), but you'll get full control over the portrait size:
This is actually fairly straight forward. In CharacterSelectionScript.cs, look for the OnGUI() method, and find these lines:

            foreach(CharacterInfo character in UFE.config.characters){
                float xPos = 96 * xCount;
                float yPos = 114 * yCount;

                xCount ++;
                if (xCount == gridColumns){
                    yCount ++;
                    xCount = 0;
                }
                GUI.DrawTexture(SetResolution(new Rect(xPos, yPos, 86, 105)), character.profilePictureSmall);

In the last line, see the 86 and 105 embedded inside the GUI.DrawTexture() call (it's in the Rect() part)?  That's the size of each portrait.

Near the top, the xPos and yPos are the size of the character portraits with a buffer.  And we know the portrait size, so we can also work out there's a 10 pixel buffer for width, and 9 pixel for height.

I suggest exposing these variables to the inspector to make it easier to edit on the fly.  Since these are done in OnGUI(), you can change them on the game object's inspector while the game is running too, to see what values suit best.

Good luck!  I'd love to see your screen shot of 75 characters once you get them all in!  Filling in the roster must be tedious O_O