Tank health
Get a pH and ammonia test kit at the very least so that you can monitor your tank's health. Remember to do your weekly water changes. When you first start out a tank it seems very hard, but as time goes by you get used to it and learn how to handle situations.
Fish inches
The general rule of thumb is 1 inch of fish per gallon of water. If you have 8 white clouds, that's 12" of fish (they are 1.5" long) - you said you have 4 other fish, too, so provided they are also little fish, you should be just fine.
Missing White Cloud Mountain minnow
It sounds like you've already looked a little; there's no need to tear the tank up looking for a fish that's probably dead. Watch the area around the tank for anything that smells (perhaps the fish jumped out and died). But the most likely thing is that he died and his corpse is somewhere in the tank. It will turn up eventually, it probably got lost in the plant or in decorations. Wait for it to turn up, and in the meantime watch your pH and ammonia levels carefully (the rotting corpse will hurt the water quality); do extra water changes to keep the ammonia levels low if you need to.
Plant
I had a lot of trouble even with the hardiest of live plants, and came to the conclusion that to grow them, you really have to understand the tank cycle, how the plants work, and be in-tune with the different chemicals and additives the plants need to grow (perhaps even a different type of lightblub). There can also be issues with the water you're using, too. The best advice I can give is to do lots of reading and research on planted aquariums, buy the additional chemical tests you will need, and plan out what your tank needs for plants to thrive.
