Yes you can.
You will need W2 Mate and W2 Mate Option #4 (Electronic Filing). This allows you to create SSA and IRS approved Electronic Filing (E-File) submissions for W-2 and 1099 forms.
W2 Mate generates the W2 e-file that you will upload to Social Security Administration.
You will only need one PIN with the SSA to file for all 20 clients.