If you close a tweet you are composing you‘ll have the option to save it as a draft. You can create an unlimited number of drafts. Drafts are accessible from the gear icon in the compose view, but you can also quickly bring up your last draft by holding down on the compose menu icon.
If you want to create tweetstorms or chain together multiple tweets as replies, Topics are for you. You can access Topics from the gear icon in the compose view. Hit the add button, enter a title and you can optionally append hashtags automatically at the end of each tweet. Now each tweet you create will automatically be a reply to the previous tweet until you close the Topics header.