Yes, you have one for each color of the palette. You just have to worry about the design of the logo and its proportions. Take a look at the wiki
Is there any guidelines to follow drawing the icons?
It is convenient to visit the Google Material Design website. The templates include most of the items, so you will not have to worry about them, but this site provides much interesting information you should follow when designing.
The update don't aply the new icons
Probably a CM13 bug (?). You can solve it applying the stock icon pack and then applying again TwelF.
I had a themed icon, but a recent update of the app reverts it to the original one
Probably the upstream changes the activity name into androidmanifest.xml. You can compare the activityname using Turtl with the code of appfilter. If the activityname have changed in fact, then you can try to do a MR with a patch, or request for a fix trough Issue tracker
An app crashes when attempts to create a notification icon with the theme applied.
This must be fixed on the upstream adding a proper (smaller, all-white) notification icon setSmallIcon. Is possible that for Android 4 (CM11) can be added another colored icon (SetLargeIcon) to be shown in the ticker and notification.
Please contact the upstream and report the bug. You can provide them more information referring to these links, in which some apps have already fixed this problem. And If you can, report relevant information here for completing the documentation about this problem.: