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Yes, you can, but you can't put them all in series. Each LED drops about 2v when forward biased. Best you can do on 9v is to put 3 LEDs in series with a 150 ohm resistor for a 20ma current. Make up as many of these 3-LED series circuits as you need and then put these circuits in parallel across the 9v. A higher-value resistor will lessen the current, which might be a good idea for that wimpy 9v battery.
If you have to have an exact 20 LEDs, you could put 4 of them in series with the resistors, but if the LEDs happen to drop more than 2v, they may not light up. Red LEDs tend to drop 2v; other colors will drop more; weird colors such as white, violet or blue can sometimes get a little goofy and drop lots more, so you might not even get 3 in series to work.
Some folks will say that you can just put all the LEDs in parallel with each other and them put them in series with a current-limiting resistor and the 9v, but that's not the best design since the LEDs will vary in their voltage drop and some will hog more current than others and possibly be damaged.
YEAH I'm having trouble with the BLUE MCD IS 2600 but the JUMBO RED has MCD 5000 WHAT if each lead from each LED is connected to the battery? it seems brighter..... I have a few resisters and capasiters IF THEY would help I don't have a clue.... most of the LED ARE large mcd'
You mention MCD. Are you trying to match brightness of each LED or are you just trying to get them all to light? To properly design your circuit, it would be helpful if you knew the voltage and current rating of each LED. If you don't have the information on the LEDs, Dean's advise would be the best you can do. Oh, I almost forgot, if you connect each wire from each LED directly to the battery, your LEDs will burn out - the 2 volt ones will probably go out first.