Do I have to buy an add-on for every show, or just the ones I want it on?

You buy an add-on once, and it covers every show on that licence.

Add-ons attach to your licence, not to an individual show. Buy the Email Editor once and it is available on every show on that licence, including shows you add afterwards. There is no second purchase and no per-show charge.

There is nothing to enter to direct the purchase. The add-on lands on the licence you bought it on, and covers whatever shows are registered to that licence — then and later.

What that means on each plan:

• Launch through Pro — one show, so there is nothing to think about.
• Multi-Show — up to three shows, all covered by the single purchase.
• Studio — every current and future add-on is included on every show. Nothing to buy.

You choose which of those shows it appears on. Buying is licence-wide; switching on is per show.
Every entitled add-on card in Settings → Add-ons carries an Active on list with a switch for
each show, so you can run an add-on on two shows and leave it off on a third. That is a display
choice, not a billing one — you have already paid for all of them, and turning one off refunds
nothing and saves nothing. Turning it back on restores it, with whatever you set up inside it
intact.

Upgrading your plan widens what an add-on covers, at no extra cost. Add-on entitlements are not fixed to the show count you had when you bought them — they follow whatever your licence allows now. Buy the Email Editor on Pro, then move to Multi-Show, and it covers all three shows from the next licence check. There is nothing to re-purchase and nothing to re-enter.

Add-ons need an active plan behind them. The free WordPress.org plugin cannot use them; a free Launch account is the minimum.