You will gain control of a creature for each of the X Keigas you created and at most one fight will happen. You decide the order.
When the Keigas enter the battlefield, each of them triggers the fight ability, then the legend rule happens. You have to choose any one Keiga you control (original or one of the tokens) and the rest die. This is a state-based action.
704.5j If two or more legendary permanents with the same name are controlled by the same player, that player chooses one of them, and the rest are put into their owners’ graveyards. This is called the “legend rule.”
This happens after the fight abilities trigger, but before they enter the stack:
704.3. Whenever a player would get priority (see rule 117, “Timing and Priority”), the game checks for any of the listed conditions for state-based actions, then performs all applicable state-based actions simultaneously as a single event. If any state-based actions are performed as a result of a check, the check is repeated; otherwise all triggered abilities that are waiting to be put on the stack are put on the stack, then the check is repeated. [..]
You are left with one Keiga; the other X Keigas die and trigger their death ability. Both the fight abilities and these death abilities are waiting to enter the stack.
State-based actions area checked again and, if none apply, all the triggered abilities enter the stack in the order of your choice. You pick the relevant targets as you put each ability on the stack.
603.3b If multiple abilities have triggered since the last time a player received priority, the abilities are placed on the stack in a two-part process. First, each player, in APNAP order, puts each triggered ability they control with a trigger condition that isn’t another ability triggering on the stack in any order they choose. (See rule 101.4.) Second, each player, in APNAP order, puts all remaining triggered abilities they control on the stack in any order they choose. Then the game once again checks for and performs state-based actions until none are performed, then abilities that triggered during this process go on the stack. This process repeats until no new state-based actions are performed and no abilities trigger. Then the appropriate player gets priority.
Then the stack starts resolving. If you chose to keep a token Keiga for the legend rule and the other creature is still being controlled by your opponent, that fight can happen. No other fight can happen because the Keiga it originated from will be dead to the legend rule.
701.12b If one or both creatures instructed to fight are no longer on the battlefield or are no longer creatures, neither of them fights or deals damage. If one or both creatures are illegal targets for a resolving spell or ability that instructs them to fight, neither of them fights or deals damage.
So in total: If you created X Keigas with Aggressive Biomancy, you take control of up to X creatures because X Keigas will die to the legend rule. If you kept a token Keiga for the legend rule, it will fight the other targeted creature if it’s still controlled by your opponent. You choose the order of control changes and the fight.