It all depends on how long you intend to stay in each city! Please note that there is compulsory reservation on all high speed trains in Italy. Please read the guidance on the Eurail or Interrail sites.

I will be staying:

flying into Milan- 2 nights

Rome- 2 night

Naples- 3 night

Venice - 2 night

our flight is back from Venice

That makes 3 days travel so that’s all you need! Make the reservations asap if travelling in August.

do I need to buy the pass now before i reserve the seats 

what if there are no seats and we book passes?

Seat reservations are completely separate from the passes so you get these first. Once you have the reservations buy the passes. Look at the reservations section and advice on this site about how to resere in Italy.

Here is info about how to make reservations for Italian day trains.

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105?postid=9551#post9551

IF money is more imptt as time: check what normal tickets may cost on those days-IT is not overly expensive as UK so you might be well surprised. R->N can also be done on much cheaper regional treni.

Then there is also private competitor Italo trains-most often a little cheaper=does not take passes.

IF you decide on pass: READ-as direct RES cannot be done via trenItalia site.