Hi. Hope this helps.

re: "...Also how cans of r134a do I need to fill this a/c unit,1991 240...."

Unless your system has been converted after it was built, a 1991 takes the old R-12. The A/C system wasn't designed for R-134a until the 1993 model year. And you can't mix them, because they require entirely different lubricants in the compressor along with some hoses and seals, etc.

The next issue is confusing. In the title you wrote "...A/C fan wont kick on?...", but in the text box you wrote "...I charge up my A/C,but this time I cant get the a/c to come on...."

Are you saying that the A/C isn't working (that is, it's not cooling)? Or are you saying that you expect the auxiliary fan (ahead of the radiator) to come on as an indication that the A/C is working? Or, are you referring to the circulation fan in the dashboard that is not pushing air through the cabin?

To tell if the compressor works, just look at its pulley. If the compressor turns on, you should see the clutch engage with a loud click and the pulley begin to be driven -- but you probably know that. So I'm guessing that the inactivity of the auxiliary fan is what has you confused.

Realize that your auxiliary fan (ahead of the radiator) isn't supposed to work 95% of the time that the A/C is on. That fan is only there for "extreme" conditions -- it's driven by a relay, as you guessed, but the relay is controlled not by the A/C compressor's activity, but by a pressostat (pressure controlled switch) in the A/C line. Only when A/C line pressure is very high is the fan supposed to work. You can test the whole fan control system by pulling the plug off the pressostat and putting a paper clip across the contacts -- then the fan should come on with a roar.
But remember that unless it's 95-100 degrees outside, and the car is sitting and idling with the A/C turned up high, the fan isn't going to turn on.

Good luck.