I don't know the answer to your question, but I do know of a reasonable way to test this.
1. Cheat kill swords ("killsw01") for your party. This ensures that you will quickly kill the mercenaries and won't die. (Note that this weapon is not balanced for normal play, so I would only use it to test things like this, or if you don't care about the gameplay at all.) (Also, if you are playing the Enhanced Edition, there is also Story Mode, I believe.)
2. Disable all auto-pause settings.
3. Turn the party AI on.
4. Once the mercenaries are after you, just let the game run. If, after an hour or so, mercenaries are still coming (and the game isn't paused), then it is probably safe to assume that the mercenaries are infinite.