Short answer is, you can't create 1 Safe Harbor statement for multiple Schedule E rental properties. Lacerte will create a Safe Harbor statement for each rental real estate activity that you check the Safe Harbor box under the QBI section. The Safe Harbor Statement is a sample provided because users requested it. It does not e-file with your return.  For the election to be valid, the taxpayer must sign the  statement(s) and then you scan and attach to the PDF the e-filed return. Since there isn't specific guidance, you could a) have your taxpayer sign 1 statement and throw out the additional statements (you may want to modify the description to include 'all rental properties') or b) have the taxpayer sign multiple statements and scan them back into 1 PDF to attach.  

I suspect this may change in future years as Treasury simplifies and clarifies the regs- we'll see.