How much AH should my battery has for my sound system (3 Amplifiers)?
The peak current you can draw from a battery is not directly related to its capacity in Ah. For instance, your existing 45Ah battery starts your car, and the starter motor probably draws north of 200A for a few seconds. If it does that, it will clearly deliver 68A peak.

So the real question is, how long will the battery and the alternator power the amplifiers at your listening level?

There are too many unknowns to give a real answer.

If the question is, 'I have 50Ah, how long can I deliver 68A for?', then the answer is 50/68 = about 45 minutes. However, that wasn't the question. Amongst the confounding factors are:

Your amplifier will not draw 68A continuously. It may do that at the peak, but it will usually be much less. 

Your alternator will deliver some current so the battery will last longer ... unless it's a cold night and you're using headlights and all the electric heaters you can.

You would not want to drain a 12v car battery fully. Going regularly below 50% capacity shortens its life, and if you flatten it with music, you won't be able to start your car in the morning.

You mention in a comment that maybe you just wouldn't turn the system up loud. However, unless your electrics are instrumented, you won't know how loud you can set it.

My recommendations are

a) Install the system. 

b) Plan for the continuous load to be supplied by the alternator, so the battery evens out the supply between traffic stops and loud songs.

c) Install a large 'audio capacitor' at the amplifier terminals, which evens out the load on the battery between the individual peak cycles of the loudest notes. This reduces the reliance of the amplifiers on very short fat cables to the battery, and the battery impedance.

d) Instrument the electrics. Install a voltmeter on the battery, and/or a current meter in series with it.

The current will give you an indication of whether you are charging or not, and by how much, but it won't tell you the state of the battery.

The voltage should be around 14v when the battery is fully charged. If it goes below 11v, turn the system off, you've really over-done it. If it's in the 12-14v range, you are using both battery and alternator, and the battery is not fully charged. I wouldn't want to see less than 13v shown on a voltmeter, especially towards the end of my journey when I had to start in the morning.