I'm sure there is a better way to do this, but you could declare a variable outside the loop and increment it inside the loop (or use it in lapply) like so:

lapply(1:200, function(i) {
sample_name <- paste0("sample", i)
sample_file <- read.delim(paste0(sample_name,".hg38_multianno.txt", header = F)
colnames(sample_file)[1:10] = c("Chr", "Start", "End", "Ref", "Alt", "Func.refGene", "Gene.refGene", "GeneDetail.refGene", "ExonicFunc.refGene","AAChange.refGene")
sample_file <- sample_file[30:nrow(sample_file),]
sample_file <- sample_file[,-c(11:87)]
sample_file$Tumor_Sample_Barcode = sample_name
write.table(sample_file, file=paste0(sample_name, ".txt"), row.names = FALSE, sep = "\t", quote = FALSE)

sample_file.maf = maftools::annovarToMaf(annovar = paste0(sample_name, ".txt"), 
                                         Center = NULL,
                                         refBuild = 'hg38', 
                                         tsbCol = 'Tumor_Sample_Barcode',
                                         table = 'refGene')

write.table(sample_file.maf, file=paste0(sample_name, ".maf"), row.names = FALSE, sep = "\t", quote = FALSE)
})

While it is recommended to use apply function instead of for loop in R (like what Ram did), I understand the fucntion behavior using for loop much more better!! Here is a for loop solution :

files = list.files() # assuming you have only the intended files in the current working directory
n =  #column number of the "Tumor_Sample_Barcode" in your files

for (f in files){
  tmpDf<- read.delim(files[f], header = F)
  tmpDf <- tmpDf[30:nrow(tmpDf),]
  tmpDf <- tmpDf[,-c(11:87)]
  tmdDF[, n] = substr(files[f],1,7)
  write.table(tmpDf, file=paste0(substr(files[f],1,7), "txt"), row.names = FALSE, sep = "\t", quote = FALSE)
  tmp.maf = maftools::annovarToMaf(annovar = paste0(substr(files[f],1,7), "txt"), 
                                   Center = NULL,
                                   refBuild = 'hg38', 
                                   tsbCol = 'Tumor_Sample_Barcode',
                                   table = 'refGene')

  write.table(tmp.maf, file=paste0(substr(files[f],1,7), "maf"), row.names = FALSE, sep = "\t", quote = FALSE)

}