How to start homebrew Apache on boot?
If you run:

brew services


The last two lines of the output gives the answer to your question:

If sudo is passed, operate on /Library/LaunchDaemons (started at boot).
Otherwise, operate on ~/Library/LaunchAgents (started at login).


So, to start apache at boot run:

sudo brew services start apache2


and to start apache at login run:

brew services start apache2