Is a 20 watt amplifier enough for this space?

In a word, no. The Orange Crush 20RT makes a fine little practice amp, but doesn't have what it takes for a mini-PA substitute. Tonally, it doesn't have the wide range needed for all instruments. One of the cool things about small guitar amps is that they start to distort naturally at about half volume. On a guitar, that's desirable...not so much for music playback. Then there's the tiny 8" speaker which can't move enough air to fill a big room.

If you just want some background music that the closest tables can hear, you might get away with it....try it at home first to see how it sounds. If you want everybody to hear it, and maybe dance a little, you have to bite the bullet and rent a small PA. Hey. it's your big day, why risk spoiling the party by trying to save $50 to rent a small PA and microphone (for announcements, etc)?

I wouldn't think there'd be enough volume but just as importantly playing music from a phone through a small guitar combo like yours will sound terrible. 

Rent a suitable sound system from a local source. 

Need at least 100 watt per channel amplifier with a large good quality speaker system as its output.

you are correct. I used to have a car amp that puts out around 20 watt and it was not loud enough for me even inside the car. My first home amplifier was a Japanese amp around 40 watts per channel but it was unable to handle my low impedance Infinity speakers and died.  An amp that is not powerful enough will produce loads of distortion and the music will sound awful and the speakers can be damaged permanently as well. I now have a Harman Kardon 50 watt amp that I use at home. It is good enough for me and my Infinity speakers. It is generally a good idea to buy an American made or designed amp than a Japanese designed amp because American designed amps have beefy power supplies that can handle low impedance exotic speakers like my infinity. My infinity can have an impedance as low as 2 ohm. Japanese amps are made for speakers that are 4-8 ohms impedance, and they can shut down if given a low impedance. A Japanese amp can put out as little as a few watts when driving a low impedance speaker and may shut down, even if it is rated at 100 watt, as my Japanese amp was rated. It kept shutting down and it got damaged permanently. In contrast my 50 watt HK can handle the music at the rated 50 watts through my Infinity speakers without shutting down. I would recommend the HK 3700 which is rated at 85 watts and costs $250 now (on sale) over a Japanese amp that costs $400 or more, even if the Japanese amp is rated at a higher wattage at 8 ohms.

Generally a 200 watt amplifier is plenty good enough for most purposes because they provide the headroom to prevent speaker damaging distortion. However, it also depends on how sensitive the loudspeakers are. Most speakers can put out around 90 db for 1 watt, although some may put out less volume. The reason 100-200 watts may be needed is because some musical peaks can reach that high in power demand, if only for a split second.  Those that can reproduce deep bass also demands more power. At the very least you should consider using an amp around 100 watts or so, at least 50 watts, not a 20 watt amp. It does not matter where you put your amp. If the amp is located a long distance from the speakers, you need thick copper wires to reduce the signal loss.  Get some 10 gauge speaker wires instead of the 16 gauge ones many people use.  If the stereo store charges a lot for 10 gauge speaker wires, you can get some 10 gauge electrical wires at Home Depot.