substitute number with sed

I want to automatically automatically substitute the "number of levels" of the pedigree in a blupf90 parameter file. For that, I keep a basic.par:

DATAFILE

data

NUMBER_OF_TRAITS

           1

NUMBER_OF_EFFECTS

           2

OBSERVATION(S)

3

WEIGHT(S)

EFFECTS: POSITIONS_IN_DATAFILE NUMBER_OF_LEVELS TYPE_OF_EFFECT[EFFECT NESTED]

1 1 cross

2 30000 cross

RANDOM_RESIDUAL VALUES

0.7

 RANDOM_GROUP

           2

 RANDOM_TYPE

add_animal

 FILE

ped                                                

(CO)VARIANCES

0.3

then I read the actual number of levels in ped:

# get number in pedigree

n=$(wc -l ped | awk '{print $1}')

in my example, 2800

and I use sed to modify the 30000 to 'n':

# change 2nd number in line 13 to n 

sed  '13 s/[0-9]* /'$n' /2' < basic.par > new.par

I am not good with regular expressions, but I believe this means "in line 13, any number ([0-9] repeated from 0 to infinite times -that's what * means' and only substitute the 2nd occurrence".  The spaces in the regular expression are by trial and error :-(  
The little that I know about sed comes from the Grymoire .