1. I don't play computer games much.

2. I don't play computer games a lot.
3. I don't play computer games as much as I used to.
4. I don't play computer games as much as I used to do.
5. I don't play computer games as much as I used to play computer games.
6. I don't play computer games as much as I used to play them.
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I have written down possible forms. Can we use all the sentennces?

1, 2, 3, and 4 are commonly used, and maybe 6.

5 is not used. It's too repetitive.

Yes, all six sentences you provided are grammatically correct and can be used interchangeably to convey the same meaning. They are different variations of expressing the idea that you don't play computer games as frequently or extensively as you used to in the past. The choice to use one sentence over another would depend on personal preference or the context in which you are using the sentence.