user1760: I have two questions, both about sendmany.
First, is there any limit on how much transactions can I send with sendmany? (Apart from bash's limit with maximal parameter length.) Is sending about 500 transactions OK?
Second, what will be the fees used when I send a lot of transactions (and thus, probably creating a big transaction)?

user6024: You can calculate your transaction size before sending the transaction. Here's some more info on that:
How to calculate transaction size before sending
A single transaction isn't limited to how many outputs it has, rather, there are some limits to how big a block is. You can read more here:
How many different destinations can a single transaction have?
EDIT:
After calculating the size, you can further calculate fees by using this method (it takes into account some code in main.h): 
What is the exact formula for calculating transaction fees?

1760: <p>I have two questions, both about <code>sendmany</code>.</p>

<p>First, is there any limit on how much transactions can I send with sendmany? (Apart from bash's limit with maximal parameter length.) Is sending about 500 transactions OK?</p>

<p>Second, what will be the fees used when I send a lot of transactions (and thus, probably creating a big transaction)?</p>
 6024: <p>You can calculate your transaction size before sending the transaction. Here's some more info on that:</p>

<p><a href="https://bitcoin.stackexchange.com/questions/1195/how-to-calculate-transaction-size-before-sending/3011#3011">How to calculate transaction size before sending</a></p>

<p>A single transaction isn't limited to how many outputs it has, rather, there are some limits to how big a block is. You can read more here:</p>

<p><a href="https://bitcoin.stackexchange.com/questions/5341/how-many-different-destinations-can-a-single-transaction-have/5343#5343">How many different destinations can a single transaction have?</a></p>

<p>EDIT:
After calculating the size, you can further calculate fees by using this method (it takes into account some code in <code>main.h</code>): </p>

<p><a href="https://bitcoin.stackexchange.com/questions/3400/what-is-the-exact-formula-for-calculating-transaction-fees">What is the exact formula for calculating transaction fees?</a></p>
