There is a parts manual you can access on here but I had a look for you, they list them as individual seals or a piston set i.e. 1 seal & 1 dust seal and you have 2 different piston sizes on an mc21.

P/n: 06451-166-405 & P/n: 06451-GE2-405 you would need 2 off each kit to do one caliper plus 2 off P/n: 45103-MR7-006 these are the small seals which seal the 2 caliper halves. They arent KV3 part numbers so are probably common to other models so your dealer should be able to get them no problem.

Be sure to inspect the pistons as well and replace them if they are corroded, the corrosion can tear the seals and cause them to leak which will only end in tears!