How many deep do we need to go for ST gunners, Blockers and Rushers?

You should have 10 in each - they fill you non-kicker/non-returner players on kickoffs, so on those plays there will be 10 of each on the field.

plus subs for injuries?

Yes, the injury subs will act here as they would elsewhere.

Thanks.. also.. the depth charts are jumping around so they aren't in two rows.. I now have 3 rows