Six years ago when my daughter’s school organized FLL, there were too many for one team. They split up guys/girls (numbers were ~equal). The girls went on to be a very successful team (Dragon Devils, Michigan, in case anyone remembers).
Usually a school doesn’t have the problem of too many participants for one team - I’d guess there would have to be somewhere between 50 and 100 team members before a split would become necessary (depending on team dynamics, roles, particpation levels, etc.). If you have to do a split and there are enough girls, then you can consider it.
Unfortunately there is another reason to set up a new all-girls team - if on the existing team the girls are relegated to background roles. All too often it happens, not just on FIRST teams, that the stereotype of “girls can’t do Math/Engineering/Science” prevails. If girls can’t break into leadership and technical roles on an existing team, they should explore setting up a separate team, perhaps in conjunction with girls from neighboring teams who feel the same way. They would have to find separate funding sources, mentors and build locations. But it could be done if it is needed. Just the threat of breaking off might give the existing team the wakeup call that it needs - “Hello, 21st Century calling!”