https://www.db.yugioh-card.com/yugiohdb/faq_search.action?ope=5&fid=14069&request_locale=ja
https://yugipedia.com/wiki/Card_Rulings:Summon_Limit
If the activation of "Monster Reborn" is negated by "Magic Jammer", then it does not count as one of the two Summons allowed by "Summon Limit". (You can Summon two more times this turn.) If the effect of "Monster Reborn" is negated by effect of "The Fabled Unicore", then it counts as one of the two Summons allowed by "Summon Limit". (You can Summon one more time this turn.)
If your activation of a card or effect that would Summon has been negated, you are not treated as having attempted to Summon.
If your activated card or effect resolves negated, you are still treated as having attempted to Summon.