I usually use turbo tax to fill out 3-4 tax returns for my family, how do i do that with the on line turbo tax ?

If you are using the On-Line version, one of the features is that the  user ID/account can be used for ONE and only one tax return --e-filed or printed/mailed.  (Has always been this way)

If you need (or try) to prepare more than one return you MUST create a new user ID/account and pay for EACH return  --if you want to use the on-line version.  The same e-mail address can be used for multiple DIFFERENT account ID's.

If you try to prepare a second return using the same ID you will overwrite all data for the first return
and it will be lost forever; thus if the IRS challenges your first return you will no longer have a basis
to respond to questions.

Starting another return (with the Online version)
https://ttlc.intuit.com/questions/1894512-how-do-i-start-a-second-return-in-turbotax-online

For simpler returns like for the kids try the FREE FILE option thru the IRS :  https://www.irs.gov/filing/free-file-do-your-federal-taxes-for-free