What is the exact number for ngk spark plugs that can last on toyota hiace 3y engine

HT lead could be faulty, plug cap etc.

no . there is only one plug that fits into that engine
the plug is governed by the thread length of the plug, the seat style , (washer or taper) and hex size
Now plug make is of no matter as any brand will fit if the mentioned criteria is met however Japanese engines perform best on NGK or NSK plugs
Now what you are probably trying to ask is this
there are a number of heat ranges in the one plug that you can use example is BP5ES , BP6ES, BP7ES, BP8ES
all the same plug but with different heat ranges
The heat range is governed by the length of the ceramic insulator in the plug and is used to control the heat flow away from the electrode
when running a hot engine a cooler heat range is required , when running an oil burner engine a high heat range is required to stop fouling of the plug
Lastly the cost of plugs varies for cheap plugs that will last the life of the engine , to very expensive platinum plugs that will maintain spark at extremes of operation and last several engines

probably on firewall
best change alternator to modern one

NGK or Denso brands
but the size depends on which year and motor you have

same as factory plugs call parts store they can tell u the exact gap

You can use the platinum plugs, they will last about 60,000 miles while the iridium last about 100K miles, that's about the only difference.

It seems that the recommended plugs are NGK BKR6EP-13 as a first choice, but that the Champion EON 2 will also work. The NGK's are platinum tipped and rather pricey but needed for the high engine running temperatures.

LPG 3Y engine toyota crown comfort automatic - fuel consupmtion is 13.30litre per 100km? How can this be reduced?

Tune up, Change oil, Wires, plugs,PCV, Air filter