Answer 1

If you want to check the age , better use a switch statement

Answer 2

You can use the nested way, such as this:

if (age > 18)
{
//code
}
else
{
if (age == 18)
{
//code
}
else
{
if (age < 18)
{
//code
}
}
}

Or you can choose to use multiple if-else statements in such a way that:

if (age > 18)
{
//code
}
else if (age == 18)
{
//code
}
else if (age < 18)
{
//code
}

Both options work exactly the same, and are both viable. If you want to be picky, opt for the latter method, idealy.

And to answer your question, these if-else statements can be used in conjunction as many times as you like, no limit.