data frame names in sequence

I've been stuck with this problem for a whole afternoon. It's silly but
totally pissed me off. I have a set of data frames with names in a sequence:
df_1, df_2, df_3, ..., df_20. Now I want to access each data frame (read or
write) in a for loop, in a way something like this:

for (i in 1:20) {
  df_i <- ######
  length(which(df_i[,7]==1))
  ######
}

I tried paste or cat ("df_", i, sep=""). But neither way works. Your help is
highly appreciated!! Thanks in advance!

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