Is there a limit to the initial investment amount?
Let's say I wanted to drop 90k into it could I?

You can do 5 years at 15k per year per spouse per child at one time.

So a married couple could dump in 150k into each child's account. A single person could do 75k.

Rustys-Beef-o-Reeno said:

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Is there a limit to the initial investment amount?
Let's say I wanted to drop 90k into it could I?

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You could do that all at once. The only question is whether or not you would exceed your annual exclusion and begin eating into your lifetime exemption. The 5 year forward gift allowed for these plans, and split gifts with spouse would easily cover 90k. If you don't have a spouse and/or have other gifting, you may exceed the $15k/year/donee/recipient limit for tax free gifting. Gift tax return will need to be filed either way.

FWIW, the lifetime gift tax exclusion is now over $11,000,000. So, you can put more in if you needed to, you'd just have to apply it to that.

There are aggregate limits for 529 contributions that vary by state. Amounts range from around $230k - $500k.

As mentioned previously, you can typically gift someone $15k in a given year without any gift tax implications (which is really just a tax filing requirement to keep track of your lifetime exemption) but 529s have a special 5 year gift averaging that allows you to accelerate up to 5 years of gifts without triggering any gift tax issues. So you could gift up to $75k this year.

But if you are married, your spouse can also make gifts, so the amount is actually $150k that a couple could fund a 529 with under the 5 Year Averaging.

However, there are caveats...
1. If you buy other gifts during the year for a child, those should be included in your $15k calculation for the amount gifted.
2. Overfunding a 529 can result in negative tax consequences, having a well thought out plan is advisable.