Looking to buy some forks for my MTB, the head tube length is 150mm so what length of steerer do I need for the forks?

You need to include the headset, stem and any spacers in your measurements. These can vary quite a bit, depending on the components fitted to your bike.
Best option would be to measure your current forks from the top of the crown to the headset top cap. This will be slightly longer than your current steerer tube, but it will give you starting point.

Add the headset and stem to the 150mm

and then a bit for comfort

As an example, my headset contributes 34mm, my stem 45mm, spacers 30mm and the steerer tube is 180mm.
So I need a steerer tube to be the total length of 289mm.

I don’t have any current forks to compare size as I am building a bike up from scratch, I have bought a Merida carbon race frame.