There are theoretical mathematical limits due to the fact that you can't count an infinite number of skills in a routine.
If we ignore connection bonus and assume a routine composed entirely of F-value skills, the start value would be 18.5 under current men's rules. (I don't know women's FIG rules well enough off the top of my head to know what the max for women would be).
On events that have G-value skills and/or connection bonuses, that theoretical limit would be higher -- how much higher would be a bit more complicated to calculate.
EDIT: Actually, come to think of it, I think most events don't even have 10 F-value skills, so you'd have to use some Es as well.
Just for the fun of it, let's ignore the limits of what humans can actually do and calculate a value for the hardest-possible men's floor routine.
-Manna press to handstand (or air flare, or any other non-acro D)
-Triple back punch front triple-full (G-F, .2 connection bonus)
-Back triple-double punch Hypolito (G-F, .2 connection bonus)
-Lay double-double punch Kolyvanov (F-F, .2 connection bonus)
-Back 4/1 punch Tamayo (F-F, .2 connection bonus)
-Penev (F)
That would give us 19.3 for a start value if I'm counting correctly.
However, these connections are lightyears beyond what is actually physically possible with human joints and muscles on a spring floor.
