Our son learned his signs in the same time frame too. By the time he was 18 months, he knew about 50 ASL signs. Children this age often don't have the fine motor skills to do the signs "just right" so they create their "own" sign that is close to the correct one. Don't worry about that....you'll know what he means. little toddlers have trouble manipulating all the fingers, so, for example, instead of using three fingers for the water sign, our son used his whole hand. He did the opposite for ball and used two fingers pointing together. it doesn't matter, they know and you will know what he is talking about.
The most useful signs I taught were several of the ones you've already taught like: eat, more milk, help-Very useful, all done, where, bath, water, cold, hot, change, night night. All of the other signs I taught really just helped our conversations! it's was amazing to be able to fully communicate with a child who couldn't yet speak! Some of the other signs we used often were: rain, flag, ball, play, airplane, car, giraffe, elephant, lion, bear,light etc... Just teach your child the signs of things he's most interested in and he'll talk up a storm!!! Have fun and enjoy.
And, as someone else posted, it is very helpful when they do begin to talk to help you understand what they mean until their words become clearer.