How many elements does $P$ have after the for loop executes?

clear; clc

birth $=0\mathrm{.}02$;

death $=0\mathrm{.}00$;

deltat $=1$;

$P(1)=1000$

for $t=1$:$20$

$P(t+1)=P(t)+$ deltat $*($birth $-$ death$)*P(t)$;

end