general questions about wordpress

Hi,

¿i would like to make some general questions about wordpress?

1. ¿how many plugins should i install in a wordpress site, to avoid a slowly webpage? if i upgrade my hosting plan, could i install more plugins? ¿ is better to insert the code inside the wordpress files than install a plugin, to improve the site´s velocity?

2. if i install wordpress multisite, ¿could i safe resources?¿my webpage will be faster?

3. ¿how many profiles should i allow to create in a virtual comunity integrated with buddypress, how could i calculated the resorce´s spent to avoid a slowly page?

i´m creating a virtual community and i need to calculate the resources, costs, and other technicals items.

thanks a lot for your ansers,

David