。
There were many excited fans at the stadium.
Literal
Baseball-field [at-には] many-[gen-の] excited-[past-modifier-た] fan [subj-が] existed-[polite past-いました].
Built on いる (for animate beings) rather than the で+いっぱいだ ('full of') pattern. 多くの ('many') is the prenominal form of 多い — note that 多い can't itself directly precede a noun (it's predicate-only), so 多くの does the modifier work.