Not many tomatoes in my greenhouse this year,usually loads by now.My mums and friends are the same.Why ?

I would think a lat of summer light to initiate flowering as its not been the brightest few months!

Or as there are generally a it could be the lack of bees or air movement in the greenhouse to pollinate the flowers, then thats why no fruit.
Gentle shake the plant and tap the flowers or poke a feather in the flower.
Toms are self fertile and this is all they need, a gentle touch or a vibration form a bee!