do you have to collect CT alone or CT + GST ?
if it is only CT you can set the default tax code for the inventory item. this tax code will be automatically shown when this inventory item is selected on any form.
for batch update of inventory items with the same tax codes, first edit one inventory item by selecting the correct tax code.
then click on the Batch Update button at the bottom right corner of inventory list.
copy the data shown to an excel sheet.
now copy the exact data shown under the TaxCode column of the inventory item you updated and paste it for the other inventory items.
now copy the whole data and paste it back to Manager in the field allocated and update.
read the below guide regarding batch operations.
https://guides.manager.io/9572