Short answer: One upvas (उपवास) = 0 ekasan (एकासन).

Explanation (simple):

• Upvas (उपवास) means taking no food for the whole fast day (no “aahaar”); in many forms only boiled/cooled water is allowed. ( jainworld.com , imjain.wordpress.com )
• Ekasan / Ekasana (एकासन) means taking food only once in the day, in one sitting. That is a different vow (one meal per day), not a full fast. ( jainworld.com , thejainfoodie.com )

So if you perform a strict upvas you do not take any ekasan that day. If you eat one meal (ekasan), you are observing ekasan, not upvas.

Tradition note: Details about drinking water during an upvas differ in practice (Śvetāmbara and Digambara customs