Your guppies look to me like they are gathering at the top of the tank; do you have a bubbler and a suitable filter? A heater? Plants? Guppies should be swimming at all levels of the tank besides when they gather to rest, otherwise if they're mostly always at the top of your tank they aren't getting enough oxygen. Once I had a group of guppies doing this from a cloudy water algae bloom, teaching me the importance of cycling my tank.

As for water changes, a water testing kit is a life saver! I tend to do more water adding than water taking at this point but in the beginning my tank would require half water changes. Each tank is going to be different depending on your water and your tank life.

Beautiful guppies, best of luck!