"I seldom see sportsmen here."

Translation: Ritkán látok itt sportolókat.

[igorjales]

If I want to use Én, where can it fit there? The way I guessed it marked as wrong

[bdori]

Én ritkán látok itt sportolókat.
Ritkán látok én itt sportolókat.

[ErikAnderson3]

The word order is unclear. I think I've understood that the adverb ritkán must be followed immediately by the verb. But is there any requirement that itt comes after the verb? Would "Itt ritkán látok sportolókat" also be correct?

[Elcsike]

yes, I think you can basically put the word "itt" anywhere in this sentence as long as you don't put it between 'ritkán" and "látok". In your sentence it suggests that there is an emphasis on the word "itt", meaning that you do see sprotsmen in other places though, but here not really.
It could also be "Ritkán látok sportolókat itt."