Not really sure what your intended output is as the first post didn't make that much sense, but here is your code, but tidied up a bit.
library(tidyverse)
Categ_ <- c("f","f","f","s","t","t","s")
Freq_ <- c(10,15,5,2,14,20,3) # extra ')' removed here
datax <- data.frame(Categ_,Freq_)
glimpse(datax)
# put the n in with the sum
datax %>%
group_by(Categ_) %>%
summarise(Sum_=sum(Freq_), n = n())
# A tibble: 3 x 3
Categ_ Sum_ n
<chr> <dbl> <int>
1 f 30 3
2 s 5 2
3 t 34 2