Currently in Helix there is no way to get the total number of subscribers to a channel without pagination through all the results and calculating the sum yourself.
There is a feature request on Uservoice to add a total though https://twitch.uservoice.com/forums/310213-developers/suggestions/37833286-add-subscriber-count-to-a-helix-endpoint so If this is something you’d like to see implemented please vote for it.