Don't forget that the electrical connectors stick out beyond the fixtures.

Also, considering the spread of the light, 24" might be just fine.

For the high light plants, I'd start with 2 x 24", as they won't be very far from the lamps.

Thanks to everyone's reply!

Here is a (really bad) picture of the main shelf and peg board. It's a work in progress, but you get the idea.
The measurement is 23 inches from top to the glass shelf.
Ikea has small 7 inch shelves which fit the peg board so I can bring small compact catts closer to the light. Will two of the 24 inches be strong enough?

Just trying to get what works best. It will be quite a distance from the light to plants, hence why a strong light may help?

Again thanks every and sorry for being a broken record. I appreciate the patience.

Talk to Jeff Young, the seller. He’ll be able to answer your questions better.

I will add one small thought. You said there would be no reflection because of the glass but; 1) glass does reflect to some degree and; 2) those white walls behind and next to the cabinet will reflect quite a bit of light. Not max intensity but don’t expect it to be dark down there either

I use a different light to sprout a lot of seeds and I have one tray per shelf and the bottom one grows with only reflected light