I think the rivets are there to hold in place for soldering.

Rule of thumb is lap 3 times plate thickness, and allow a factor of 0.8 for joint efficiency. Rivets are only to hold it together whilst silver soldering, as few as possible and don’t hammer the up tight, the solder has to bd able to wick through

For a small boiler why are you using sheet copper – rather than drawn tube ? Do you have enough heat and equipment to properly silver solder this sort of size – many a potentially good boiler has been ruined by the lack of enough heat ?

The answer to your question depends on the plate thickness and also the working pressure. The calculations need to be done to arrive at the right plate thickness and there are many books that will give the formula – for a butt strap joint you will reduce the pressure by 0.8. I take it you are designing the boiler not using a published design ? Even for private use I would strongly advise you to join a club and speak to the boiler inspector, you will get all the help and advice you need and even for private use you will know it is safe ! Good Luck.  Noel.