Count number of rows per group and add result to original data frame

You can use ave:

df$count <- ave(df$num, df[,c("name","type")], FUN=length)

You can do this:

> ddply(df,.(name,type),transform,count = NROW(piece))
   name  type num count
1 black chair   4     2
2 black chair   5     2
3 black  sofa  12     1
4   red plate   3     1
5   red  sofa   4     1

or perhaps more intuitively,

> ddply(df,.(name,type),transform,count = length(num))
   name  type num count
1 black chair   4     2
2 black chair   5     2
3 black  sofa  12     1
4   red plate   3     1
5   red  sofa   4     1

Using data.table:

library(data.table)
dt = as.data.table(df)

# or coerce to data.table by reference:
# setDT(df)

dt[ , count := .N, by = .(name, type)]

For pre-data.table 1.8.2 alternative, see edit history.

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Using dplyr:

library(dplyr)
df %>%
  group_by(name, type) %>%
  mutate(count = n())

Or simply:

add_count(df, name, type)

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Using plyr:

plyr::ddply(df, .(name, type), transform, count = length(num))