I want to mount a table top of 270cm by 40cm how many brackets do i need to place and how what kind of bracket should be used

How many brackets do I need and what kind of bracket should be used?

You need "L" brackets that are rated to support the maximum amount of weight that could possibly be placed on the shelf. Here is the logic you can use to arrive at a decision with sample numbers based on the description given.

Shelf Design
A rule of thumb is to space brackets every 40cm. If your shelf material needs more support space them closer than 40cm and adjust the calculation accordingly.

The two brackets closest to the ends can be about 5cm from each end.
Based on this rule of thumb you wind up with:
270cm (total length) minus 10cm (total distance from the ends) = 260cm
260cm divided by 40cm (target bracket spacing) = 6.5 brackets.
Since you can't have half a bracket go with 7 brackets spaced 37.1cm apart (260 divided by 7)

Selecting a Bracket
Select a bracket that is at most 5cm shorter than the width of the shelf. In this case 35 cm to 37cm should work. Shelf brackets are rated to support a particular weight.

If the 50kg you put on the shelf is distributed evenly across the shelf you would need brackets that are rated for about 8kg each (50kg divided by 7 = 7.14)

If the weight will rest on one bracket you'll need brackets that will support the full weight.