I know for sure that neither Lacerte nor the IRS has any such limitation. I find no documentation saying that ProConnect does - what are you seeing?

(If you're looking at something like this - https://www.taxact.com/support/313/2024/irs-limit-on-electronically-filed-tax-returns?hideLayout=Fal... - what it says is that consumers, as opposed to professionals, can only submit 5 separate e-filed 1040s. If you're preparing more than 5 returns, and you want to e-file all of them, you have to get an EFIN and a PTIN.)