There are now 2000 addition worksheets and 2000 subtraction worksheets available (free to print) on this site (each in 20 sets of 100 worksheets).
Ideally each child should be thoroughly comfortable with addition (to a level well beyond the first 100 worksheets) before progressing to subtraction, multiplication, division and fractions, in easy stages. Free worksheets for each of these processes are now on line (see further down this page).
The aim of the further sets of addition worksheets is not "endless drilling" (which tends to be fairly counter-productive) but simply a limbering up exercise, maybe 10 sheets first thing at the start of daily lessons, to build mental agility and reinforce the bonds between numbers.
Most kids will race through 10 worksheets in a very few minutes, and in class situations they can mark one another's sheets to minimise the load on teaching staff. Whilst technically the first 100 addition worksheets are part of the "key stage 1" learning process, the supplementary sets are useful for older children whenever there is a need to "get their minds working" and to ensure basic confidence with essential number skills is in place.
Links to all 20 sets of 100 are given below: