Although apply to each has a setting for changing between parallel and sequential mode it does not seem to work with approvals.
I think it is a key function of flows to be able to send out many approvals - confirmation for meny individuals.
However approval (or send email with options and send approval mail actions) seem to process such a scenario in a sequential manner. (sending out the approval mail just afer answering the previous..)
See also:
https://powerusers.microsoft.com/t5/I-Found-A-Bug/Apply-for-each-Parallel-for-approvals/m-p/57357#M1...
This has been completed. You can read more here: https://flow.microsoft.com/en-us/blog/gateway-ha-increased-apply-to-each/