It all depends how many splits you need.
The following configuration would provide you with Swim Time, TR1, Bike Time, TR2 and Run Time.
1 main system for Start
1 backup system for start
1 Main system for your SwimIn/RunOut transition
1 backup system for your BikeOut/BikeIn transition
1 Main system for your Finish line
1 Backup system for your Backup line
Start systems helps for counting how many racers are entering the water as well as if you want to be able to provide a chip time results. If you want to limit your cost, you can avoid the start systems and use a scratch time for all racers, but you will not know how many racers entered the water.
Backup systems at both side of the transition can be also removed if you agree with a % of racer not having their Split time.
I would strongly recommand that you keep Finish line Main and Backup.
You will have to choose which timing system you want to use too, all major manufacturers have chips which works well in the water.