How do I explain to my parents that sometimes I need space and I want to be left alone, without giving them the impression that I don't like to spend time with them as a family?

Explain to your parents that you enjoy spending time with your family however you also like to spend time on your own. Be open and honest with them and explain that you may need some head space from time to time. You could emphasise to them that this by no means reflects you not wanting to spend time as a family, just that you benefit from having some alone time. You could always spend time with your family first and then ask for some alone time. That way you have spent time with them first and foremost and they are less likely to feel pushed out.

It sounds like you already have a great start to the conversation based on how you phrased your question! Approaching the conversation with kindness and asking for an open dialogue to talk about this with them could go a long way. You could even consider how to share your worry of how you might make them feel by asking for this. By being kind, honest, and considerate of their feelings, you are showing them more of who you really are, and it can ultimately deepen your relationship with your family. You might even want to jot down the things you want to say ahead of time, especially if you're not used to having conversations like this with your family.

tell them how you feel tell them exactly what you told me. they well surly understand. everyone needs a little time by them self just remember they are there to help you when you need help

Explain to them that sometimes, you need some time by yourself to reflect and to relax. This is best way to let them know. If they have a problem, then explain you do want to spend time with them, but you also need your space away from everything and everyone.

It is okay if you want your personal time. You just have to be honest with them tell them you want your personal time but that does not mean don't want to spent time with them it just means you need some space for yourself.

In this case, schedule time to interact as a family and then schedule time alone. That would be much easier than trying to explain why you want to be left alone. That way the won't feel bad.