its a hard balance, bots dont just go and drop wards every time they have them, nor do they generally buy wards instantly every time they are on sale.
you will quite often see a bot with 1 or 2 wards, with 1 or 2 wards still in the shop.
if you make them buy less wards, then they will place less wards and no bot warding script made yet gets more than 3 wards down at a time, so it is pretty much have only wards you put down +1 from bots, or leave it to the bots and get 1-3 wards down at any one time.

BTW, I'm overriding the ward placement and make the bots buy wards every time it's available. So it's more likely the team observer ward will be out of stock all the time. But I made bot response your ping in the latest update and give you ward whenever you ping near the bots that have ward.

how are you making them buy more often? in the current script there is nothing stopping them buying as soon as it is ready to buy, but they dont always.

It's on item purchase. Because I use this logic :
if( GetItemStockCount( "item_ward_observer" ) > 0 and
npcBot:GetGold() >= GetItemCost( "item_ward_observer" ) and
GetEmptySlotAmount() >= 2 and
GetItemCharges("item_ward_observer") < 2 and
not HasItem(npcCourier, "item_ward_observer")
)
then
npcBot:ActionImmediate_PurchaseItem("item_ward_observer");
end
So bots will buy wards every time it has stock, have gold to buy it, when they have empty main+backpack slot more than or equal 2, have ward stock in inventory less than 2, and courier not delivering their wards. So buying wards can be shared between 2 supports. With that logic bots will not buy ward if it has less than 2 empty slot or have ward charge equal 2 or courier is delivering their wards.

Yea that is exactly the same as you have always had it, which is what i have been running too.
maybe i need to pay more attention, but for me there are plenty of times during the game where wards are available, almost every game there will be 4 in the shop when i check.