You can eat a small percentage of the crop, the rest needs to be left to feed to crown/stool so that it will continue to grow and provide more rhubarb next year.

I agree with moon growe you can eat it but leave some on the crown to ensure it carries on living. enjoy.mmmmmmm

However - the stalks are all killed off by the first frost. So if you are canny you can pick any that are still useable just before the frost kills them. Not always much good for pies but OK for jam. Try them half and half with lemon Marmade if you an find any.(In case you don't know, its tinned ready prepared orange or lemon all ready for boiling with sugar for home made marmalade- you can oftern get it from Tesco) I have done this heaps of times!