There are some considerations to take into account when you are figuring out how much you should buy in to a cash game with. If you are a newer player, you should consider buying in for the minimum. This will put you under the least pressure, and if you lose it, you will not be as upset and will be less likely to go on tilt. You will also be able to watch the players and get a feel for how the game is played and maybe you will get into a good situation just because you are a short stack.
However, if you see that the player to your right – the one who is going to act before you – is a very bad player and has a very big stack, then you may consider adding more chips to your stack to have him covered. As a general rule, wait until you are the button to add more chips. There is no reason to add chips when you are under the gun and are going to have a lot of tough decisions to make. You might as well wait until you are in late position.
If you are not going to buy in for the minimum, you should sit with the maximum amount – this way everyone can be somewhat afraid of you. Buying in for something in the middle takes a lot of work to try to figure out what the effective stack sizes are against different opponents, whereas when you buy in for the maximum, all of that is much easier to figure out.