How many verb tenses are there in English?

Three simple tenses. Three perfect tenses. Six progressive forms. So, twelve.

In order:

/_Past Perfect_/_Past_/_ Present Perfect_/_Present_/_Future Perfect_/_Future

+progressive (continuous) in all tenses;

Simple Present - I walk

Simple Past - I walked

Simple Future - I will [or shall] walk

Present Perfect - I have walked

Past Perfect - I had walked

Future Perfect - I will have walked

Present Progressive - I am walking

Past Progressive - I was walking

Future Progressive - I will be walking

Present Perfect Progressive - I have been walking

Past Perfect Progressive - I had been walking

Future Perfect Progressive - I will have been walking

In addition there are two emphatic tenses:

Present emphatic - I do walk.

Past emphatic - I did walk.