There is nice function count.fields
(see help) which counts number of column per row:
count.fields("test", sep = "\t")
#[1] 1 2 3 4 5 6 7 8
So, using your second solution:
no_col <- max(count.fields("test", sep = "\t"))
data <- read.table("test",sep="\t",fill=TRUE,col.names=1:no_col)
data
# X1 X2 X3 X4 X5 X6 X7 X8
# 1 1 NA NA NA NA NA NA NA
# 2 1 2 NA NA NA NA NA NA
# 3 1 2 3 NA NA NA NA NA
# 4 1 2 3 4 NA NA NA NA
# 5 1 2 3 4 5 NA NA NA
# 6 1 2 3 4 5 6 NA NA
# 7 1 2 3 4 5 6 7 NA
# 8 1 2 3 4 5 6 7 8