Correct - initial front line is 6 units for everyone (for land battles) - whether there is a leader there or not. The side with the better leader (adding initiative + strategic rating - the top and the middle numbers on the leader counters) gets a bonus +1 extra front line unit.
Following rounds additional front line units are added out of reserve by rolling on the reinforcement table on the battleboard referencing the column with the leader's strategic rating.
After the first round, there is no limit to the number of front line units that may be in the battle. It is only governed by casualties and reinforcements. For example, if one side starts with 6 units, takes 2 casualties, and then gets 3 reinforcements, he will have 7 front line units for the second round of combat.
Hope this helps.
Mark