"How much" is like "how much food do we need"? Who is "we", and how hungry are we? Some players get by on a single 30x30 farm, some (me) like several 50x50's and keep going. No limit, really.
TC farms grow fairly densely, but slowly (as do any trees - never fast enough). Dense enough that if a caravan had to get through, it would have to work its way. And slow enough that if you don't keep building farms early, I find that I've cleared one long before the next is even thinking of being ready.
So, I like to overdo it - never enough. The safety of underground tree-cutting is a huge draw. Iirc, you can train 2 miners up to Legendary+5 in a single 48x48 area block in under 3 seasons if you first carve up-stairs, and then clear those. (You can also then carve down-stairs and get 50% more usage out of the same area, but I don't like the flashing, and down-stairs have their own pitfalls, as it were.)
For irrigation, I like to have a tunnel come up in the rough center of where I'll have the farm - always seems to do fine. I pump a pool that feeds the tunnel higher than its level, and let it bubble up - no chance of a system failure, the pump has to be working or the pressure isn't there. Re-irrigation never seems a concern due to the density.
Also, trees and shrubs compete against the limit on the max # of "things" that can grow in an area block (a single 48x48-tile biome, as distinguished on the embark Local map.) So if you can remove the shrubs, that will encourage more saplings.