50 Mhz pulse capture in 20 Mhz domain

Do you need to count the pulses or capture their time?

You will have to do your capture logic in the 50MHz domain and transmit the information into the 20MHz domain. The only ready made component I know about to transmit information from one domain to the other is the dual clock fifo. If you don't need a fifo you'll have to do the synchronization yourself, using several cascaded registers to avoid metastability issues.