house plant is yellow
Your plant is called a pothos. It is normal for the stems to be yellowish green, and in better light (they like bright filtered light but can live in dimmer postions) the leaves would have a good deal of yellow marbling.
They all are looking spindly. During this time of year (the active growth period) they need moderate water, allowing the top 1/2 inch of soil to dry out before doing it again.(Less in winter when they rest). They also appreciate a liquid fertilizer every two weeks during spring and summer.
They are not hard to root from cuttings if you wanted to look into that to thicken your planting, but they are not hard to find nor expensive if you want to plant the whole box anew.
Pothos - I didn't recall the name. We have many of them around the house. I typically cut and root to make new plants. I typically don't plant many together in the same pot - usually I put them in new pots. Would it be better to root and plant in the same pot?
Fertilizer - I do that infrequently, but will step it up a bit.
Yes, ideally you would like to see that pot full and not see the soil. They can be arranged to grow over the soil or each other before they flow over the side.
I prefer not to start new as I've got a lot of the plants all over. When I root new cuttings I typically put them into a new pot, should I plant them back in to make them more dense?
Should I take a bunch of these and put them into one pot to provide better 'coverage' and not see so much soil?
Yes, you could certainly plant your cuttings back into the original container. That will give you a more dense planting and cover the soil until the stems start growing long again. You can continue this processes of rooting and planting the cuttings.