The plants will be competing with each other for nutrients and water when they're right next to each other. It's almost always better to thin them down to 1 or 2 plants. It depends on the variety and how much you're feeding and watering them. I'm not sure how big lemon cucumbers roots get, but if they're anything like most vining plants, they'll need a lot of root space for each plant.

I thin my pumpkin plants to only 1 plant per hill (sometimes 2 if I'm doing a smaller variety and the plants are a few inches apart). Each hill is at least 5 feet from the other hills. When I started doing that, each vine produced 5-6 pumpkins each. When I first started gardening I didn't do this and I'd have 1, maybe 2, pumpkins on some plants, but usually all the pumpkins would just die and fall off after they got around 2 inches big.

Thanks, good advice. I thought that might be the case. Hope I can pull some without damage to the others. I guess they say to plant 3-4 together thinking all of them won't sprout!

When you thin them just cut the stem right above the soil line. There's a chance the roots will be tangled and if you pull them out it could damage the plants you want to keep. The roots that remain in the ground will decay to almost nothing pretty quick.

I think there's 2 reasons for over-planting: the germination rate you mentioned, and sometimes some seedlings will be more energetic than others so you can "cull the herd".

Thank you! Great advice, will do!