“There are betting shops, called bookmakers, on most High Streets”.

Can I say “… on most OF THE High Streets”, too?

Thanks once more. Ciao!

It wouldn’t make the same sense.

high street = the main shopping street in the centre of a town.
On most High Streets. (on the majority of the High Streets in the country)

If you use ‘of the’ you are likely to be talking about a selected number of high streets… the majority of the high streets in this particular selection.