I wanted to point out that the link above does not work for me!
I would add that, in general, in medieval times the power hierarchy can be really, really convoluted. A situation like "under the Duke there are six Barons, each controlling twenty knights" is too "clean" to be considered even remotely typical. Some knights hold their land directly from a king; some from a noble who holds it from another noble who holds it from another noble who holds it from the king. Some nobles have multiple lieges; some have as their liege another noble who is their vassal (i.e. noble A holds fief X for his liege B, while noble B holds fief Y for his liege A)! Also, in general what "being someone's vassal/subject" means can vary tremendously even in the same region, as laws, tradition, treaties, inheritances etc. intersect in an incredibly complex web.
Also, we have relatively little information about who held what below duke/baron level in 1220. The upside is this: you can probably make up whatever you want!
One good guideline to keep in mind is that the "initial" form of feudalism is about splitting your lands, which are too big for you to administer, between your buddies (the buddyness often reinforced through marriages). In exchange, they'll provide you with some goodies, but most of all with troops, that is the basic unit not just of power but also of status. For obvious reasons, you really, really want to ask them to provide as many troops as they can; and they want to do the same, because the more troops they provide, the better they look both to you and to other nobles (meaning less likely aggression, more likely "promotion" etc.). Since this division percolates down to the "basic" unit of feudal warfare, the knight (with a squire and a handful of loyal footmen), the typical "basic" unit of land is a "knight's fee" i.e. just enough land to maintain one knight and a handful of men at arms. This is something that varies widely, but something between 1000 and 2000 acres of mostly farmable land would be considered "typical", with perhaps a hundred to several hundred people living on it. Just to give an idea, in 1200 there are several thousand such fiefs in England.
In general, a noble does not control more than a few dozen vassals directly; this makes things too messy compared to selecting a few good, trusted buddies, and giving them large lands and the task to find how to split them. So, there's at least one intermediate level, and most likely two, between the Duke of Swabia and most of his knights. Too few vassals are, however, also a liability, since then they tend to be powerful and challenge the power of their liege (one excellent way to ameliorate this problem is to give your vassals land in a lot of "crumbles" rather than in a single chunk, so they'll have a hard time building a power base if they do decide to rebel).
Finally, the fact that, by 1200, a lot of the military obligations have been changed into "scutage" (money payments, with which the liege can hire mercenaries directly loyal to himself, or do other stuff), and a lot of the original fiefs got split and recombined passing hands as inheritances, dowries, objects of litigation etc. makes things really complicated: some knight fees have been split into fractions of fees, some knights hold multiple fees (sometimes more than some barons!), under the most a fantastic set of obligations. A typical example would then be a baron, who holds from the local duke 25 and 1/2 knight fees (split between the baron itself, 3 close relatives, 8 other knights, another duke who's the baron's vassal for 2 of those fees, a widow of a powerful knight, and the young daughter of another knight who is the ward of another baron and heirress to other lands) and 8 more fees from the local archbishop (with a tangle of agreements about supporting a local abbey); and he also holds 2 and 1/3 knight fees from the Teutonic Order, and 1 more knight fee in another kingdom; plus, toll rights on a certain road, taxation on some specific goods entering a certain town, the right to operate a certain mine etc. etc.