SQL Server 2019

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create table #number (number int, date datetime2)
insert into #number values (10, '20210101'), (20,'20210506')
2 rows affected

create table #tarif (tarif int, dateStart datetime2)
insert into #tarif values (10, '20210101'), (20,'20210501')
2 rows affected

with cte as (select row_number () over (order by dateStart) as rn, tarif, dateStart from #tarif)

select n.number*x.tarif, n.date from #number n join (
select c1.tarif, c1.datestart, isnull(c2.datestart,getdate()) as dateEnd from cte c1
left join cte c2 on c1.rn=c2.rn-1) x on n.date>=x.dateStart and n.date<x.dateEnd



(No column name)
date


100
2021-01-01 00:00:00.0000000

400
2021-05-06 00:00:00.0000000

(No column name)	date
100	2021-01-01 00:00:00.0000000
400	2021-05-06 00:00:00.0000000