Maybe I can help you understand how it works:

There are two type of heroes, exclusive and public heroes. Exclusive are unique to each player, and only accessible by them until the timer is gone("DurationOfHeroesExclusivity"), once that happens, those heroes go to the public market and are accessible by all players, and the heroes that remain and are randomly chosen between the previously exclusive ones.

So the values that would affect the heroes on market are:

SimulationDescriptors[EmpireType].xml:

SimulationDescriptor Name="EmpireTypeMajor"->DurationOfHeroesExclusivity: Gets heroes from the 'HeroExclusive' pool, makes those heroes only accessible to that player for this period of time, you can tell this because of the counter next to the exclusive heroes.

SimulationDescriptor Name="EmpireTypeMajor"->MaximumNumberOfExclusiveHeroes: Tells how many heroes are drawn from the 'HeroExclusive' pool each time the exclusive heroes go to the public market.

TradableCategoryDefinitions.xml:

TradableCategoryDefinition Name="TradableHero" -> MaximumNumberOfOccurencesOnMarket: This is the number of heroes for the public market, these heroes were previously exclusive, and are randomly chosen from the exclusive previous ones. If there are not enough exclusive to replace all the previous, they are added up to the public hero list.

TradableCategoryDefinition Name="TradableHeroExclusive" -> MaximumNumberOfOccurencesOnMarket: This is the number of exclusive heroes, that must be at least (number of players * MaximumNumberOfExclusiveHeroes) to avoid bugs.

My apologies for the lateLateLATE reply...

I use the latest gameversion v1.5.14 s3 with all official DLCs and no other mod than your EH.
I changed the files in the public simulation section of the main game folder
(i have backups and do not know how to create a full external mod) and
adapted your mod at one place (MaximumNumberOfOccurencesOnMarket)
so i still get the other additions from your mod.

I fiddled around a bit.
My goal was to have a high amount of variety to choose from regarding heroes and units.

In SimulationDescriptors[EmpireType].xml i changed:
1. DurationOfHeroesExclusivity from 15 to 0 (so i can see how it works, while testing,
without having to play an hour each time i change something)

2. MaximumNumberOfExclusiveHeroes from 3 to 100, to have beforementioned variety,
because with a quite low number, too many duplicates show up.
(some may never show up then, which i did not like)

In TradableCategoryDefinitions.xml for each of
A. TradableUnit
B. TradableUnitNeutral
C. TradableUnitNeutralSeafaring

I changed:
1. MaximumLifeTimeOnMarket from 10 to 10
2. MaximumNumberOfOccurencesOnMarket from 16, 8, 6 to 100, 100, 100 respectively
(when testing, as you know, it makes more sense to use BIG numbers,
as small changes often drown in the variance)

So far, the changes for the heroes work fine, after 25 turns or so,
i have around 150 to choose from.
Weirdly, still only 1 or 2 forgotten love heroes appear while having 2 of almost
every other hero, sometimes 3. There must be some sort of weighing, i think.

I play with the Roving Clans for testing, as they have the mercenary market right away,
so i do not have to research that every time i start a new testgame.
I do not know though, if the empiretype influences which type of heroes show up.

However, the above values still leave me with only 23 to 30 units
(opposed to the expected 100-300)
and very often, 10 of those are boarding vessels.

Maybe you can help me find out where else the max number of units is controlled/set?