How much Terrain do I need to start

Yes welcome back and good advice from Timurilank. Look at what terrain types your armies have and then start by producing the pieces allowed in that type, e.g. ploughed fields for Arable etc.

It also depends on how realistic you want your terrain pieces to be, as there is usually a trade off between realism and functionality, e.g. if you model a steep hill that actually looks steep, it may be too sharply angled for your figures to stand on it and they could keep sliding off.

I must admit I tend to lean towards the more practical approach, so I have felt pieces of different colours, green, brown, sand, blue etc and then for the area pieces I have markers which donate the terrain type. These are individual trees, or a modelled sand dune or a small piece of marsh, scrub, rocky area or building etc. These can be moved around as required to allow troop movement. These are also quite light and durable if you're going to be carrying your terrain around.

If you look at some of Tony Aguilar's battles on You Tube, on some them you'll see he has fully modelled hills, but puts them on a spare base of the same dimensions, so when troops need to move over them they can do by removing the top model.

Have a look at some of the games on YouTube or pictures and you'll see lots of ideas for the way other people produce terrain.

Finally don't forget that if you're placing the terrain your opponent will choose which edge you play from (although see the rule about waterways and roads).