If I use Falinks' "Form Ranks" attack while Area Zero Underdepths stadium is in play and I already have 4 non-Tera Pokemon on my Bench, how many Pokemon can I put on the Bench if the first one I place is a Tera Pokemon?

Only one. Since there were no Tera Pokemon in play when the attack was used, there is only one space available to fill on the Bench.

Relevant cards

SV7_88

SV7_131