I don't believe you can, but you could try marrying a sims within a family of eight.
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if this is for the pc,you can have 8 in total but for the sims 2 for the ps2,you can only have four.
Up to 8 people, or, if you have Pets, a combination of 10 with no more than 8 sims or 6 pets.
you can have 1 - 8 Sims and (for pets) up to 6, and then you can not try for baby or adopt or ask other Sims to move in all those will be greyed out.
You can have up to 8 sims per household however if you have 7 sims and 1 of them is pregnant with twins or triplets you can have more.
You can only have 8. However, if you have 7, and one of those Sims is pregnant with twins or triplets you can have more.