During the installation of the OS, I used slice 0 to 7 (except slice2 off course) and mounted some dirs to each slice.

I still have some free space left on the disk. Now I want to add a new slice.

I understood that 8 and 9 are reserved.

how can I create slice10 ? format does not allow to create slices. (only modify )

man prtvtoc
man fmthard

and don't forget to backup before doing anything risky !

solaris intel will have 10 partition starting from slice s0 to s9

we can do nothing with s8 and s9 as u said , olny we can use 7 partition as slice 2 refers to
whole disk

for example in my disk
cylinders
s0 - root -- 3 -510
s1 - swap 511 -917
s2 - refers to whole disk 0 - 9971
s3 - usr 918 - 3762
s4 - var 3763 - 4270
s5 unassigned -- probably i can use this space later
s6 unassigned - do -
s7 unassigned -do-

do format command , select ur disk and type partition and print

c ur slice 2 (identify cylinder range for eg: 0 - 9971) if any of the other slices have

end with 9971 cylinder range , for an example if u have used s6 or s7 for /export/home or opt
and ends with range 9971.
probably u can not create new slice as have already used that range , as only we can allocate space for unused cylinder range .

if u have available range , create partition using format and label it
and then construct file system using newfs and mount it under ur own directory

This is not what I read on the fmthard manual page:
format has indeed this 10 partitions (slices) limit, fmthard doesn't seem to have it.